Rating: 
Summary: Nice but too long
Review: Many reviews below state that you need a lot of mathematical sophistication to read this book. Well, the fact is that the only prereqs you need are multivariate calculus, linear algebra, a (tiny) bit of set theory, and the KT theorem (which is in the appendix anyway). Every grad student in economics should already know all that stuff, so I don't see what the fuss is about. Moreoever, the ratio of discussion to mathematics in the book is extremely high. The idea that the authors are trying to "show off" their math talents is ludicrous; take a peek in Mas-Collel's "Theory of General Economic Equilibrium : A Differentiable Approach" to get an idea of how simplified the treatment given in MWG is.There are also a lot of reviews below which criticize the text simply because they don't like microeconomic theory. This book is a textbook, not a research publication, so I don't see how that is a fair criticism. Anyway, what those reviews fail to mention is that MWG present several *negative* results about the power of microeconomic theory (see eg ch. 17) which do not typically appear in micro textbooks, so you can hardly accuse the authors of harboring some kind of ideological agenda. Anyway, I think this book is generally pretty good, although the quality varies a lot in different sections. The best section is definitely Part 4 (general equilibrium). In particular, ch. 17 is very nice. Part 1 (individual choice) is also good. The other parts are ok. Unfortunately several of the exercises in part 3 did not provide enough information to obtain a solution, which was very irritating. It is clear that the authors have not solved all the problems themselves. The solution manual is also incorrect in several places. I do like this book, but the biggest problem for me is that the theory often ends up seeming clunky and inelegant. I would like to see a more terse treatment than is given in this book, without any sacrifice of rigor. Several of the more peripheral topics could surely be omitted. Also there are a lot of examples that could go, and a lot of redundant discussions too. Some of the proofs seem to go on forever, when in fact the essentials could be communicated in a much shorter space. Chopping a lot of stuff out would help to make the book more readable and less heavy. Basically I would like to see the book written more in the flavour of Debreu's "Theory of Value". Anyway, I can't think of a better text for first year graduate econ students.
Rating: 
Summary: Excellent treatment in theoretical microeconomics
Review: The book presents a very complete treatment in theoretical microeconomics. It is rigorous but plenty of explanations and examples. Some readers complain about the lack of real world examples and the math level. Well, the book is intended for graduate students in economics and requires full command of calculus in several variables, nonlinear optimisation (Kuhn-Tucker), and some concepts in real analysis. Without, this background I understand it may be painful to read it. One advice is reviewing the mathematical appendix and using a complementary text like Simon&Blume. Once you get used to this the story changes a lot (own experience). It is true that you don't find here many real world examples, but simply the book is not intended for this purpose. If you want this try another one. In sum, if you want a good background in theory this is the benchmark (I've read Varian also and some parts of Kreps).
Rating:
Summary: Complete, well explained and concise
Review: This is a really good book for those who are taking graduate courses or comprehensive exams in Microeconomics. It covers almost everything one needs to know about microeconomics and can be used as a reference book. It sure gets into technicalities too, but one has always the option to skip them and focus on general ideas. Only that the "Game Theory" section is not the best you can find. Some complementary texts is recommended for that section.
Rating:
Summary: Helps you to gain the confidence needed for academic success
Review: Going through a Ph.D. education in economics can be a risky investment. Unless you build on strong foundations the whole thing will fall apart as soon as you get out of graduate school, no matter how fancy your dissertation was. This book provides you with the fundamental tools you need to do research and teach when you become a full-time researcher and professor, in any branch of economics, either applied or theoretical. Even if you dont apply them at this rigorous level, you will always know if you are doing something wrong just by checking the basics in this book. It also provides you with the common language economists speak all over the world, regardless ideological points of view. It is the stuff we all agree with, and the starting point of any future development in our discipline. However, if you are not planning to become a professor but a professional (a practitioner economist) better turn to Milton Friedman's Price Theory for sound intuition on how to tackle the even more difficult real-world problems.
Rating:
Summary: Solid introduction to microeconomics
Review: Just like Samuelson's introductory text was for many years a standard in undergraduate economics courses, so is MWG the canonical choice for first-year graduate microeconomics. It is a comprehensive treatment that leaves out neither the rigorous mathematics that the subject deserves nor discussions of the applications and intuition, and while the size can be intimidating, instructors generally only choose the chapters most appropriate to their own exposition. The only quibble about the book is that while it covers the 50-year-old foundations but does not go into new developments of microeconomics too much, for which I would suggest _The Structure of Economics_.
Rating:
Summary: a comment on the level of maths
Review: Most people complaining about the level of math do apparantly not realize that the mathematics used in graduate textbooks on economics is quite elementary. This means, if you don't know the tools needed to understand MWG, read an optimization text like sundaram and you will be ready. No prerequisites for that. it is funny to read all the reviews complaining about the lack of intuition. You guys do not have any idea of what science is about.
Rating: 
Summary: The stuff is badly presented.
Review: When there are Varian's book and Nicholson's book, why would Mas-Collel write another one? Especially when the author can't beat either of them. This has been my question for almost 2 years. The authors present almost every idea in the most complicated way they could find. They use many notations that are not very well adopted by 'the usual calculus' which most economics students have taken. Instead, they use many notations borrowed from Real Analysis, Set Theory, and so forth... these are Math PhD level or even higher courses. To explain the ideas, those are not necessary. Before I entered economics, I always thought economics is not rocket science but economists like to make it look like rocket science by intentionally talking in a difficult way. Mas-Collel's book proved my thought. This book is so wordy. The author can't even explain the simplest idea without wasting pages. I sometimes even doubt whether the authors really understand the 'things' (sure they do, they are from Harvard and Mas-Collel became hot in Spain). I just wish they could explain things instead of showing how difficult it is and how high the level is. I once struggled 3 days on this book trying to understand something, in the end I still had no clear idea. I switched to Nicholson's book and I understand the idea immediately and I will never forget it in my life. I wish our professor could choose Varian's book for our first year PhD course. Mas-Collel's book wasted me so much time and I learned very little from it. It tries to touch everything and can't explain anything in the way a student would like. The comparative statics part was badly written. If someone wants to set this as a standard, then the professors should require the students take real analysis (math PhD level) before micro. Because Mas-Collel uses their symbol and language a lot. I believe a true master is someone who can explain complicated things in easy ways because they are competent and have confidence in what they are doing. I bought Mas-Colle's book brandnew. Next time I move, it goes to the dumpster. I will only keep Varian and Nicholson's book.
Rating:
Summary: Pretty Darned Good
Review: Firstly, take care to read the mathematical appendix. Just get used to the notation. Once you do, I think the math is not at all hard. If you are going to be a graduate economics student, you HAVE to be totally comfortable with the math used in here. And what they teach you in "math camps" or first-year "math methods" classes goes way beyond what is used here. I think it's much more the notation that puts people off. Can anyone please point out where exactly the MATHEMATICS used in the book is arcane or inaccessible? Yes, some of the questions at the back are quite hard, but again a little perseverance and the purchase of the Segal/Hara/Tadelis review book and I think you can do well in your micro prelims. It's much more the case that MWG's presentation is very formal as is their notation. Yet once you get past this, the mathematics employed is really just multi-variate calculus and a bit of matrix algebra. Strength: chapters 13, 14,15,16,18,21,22,23. Really well laid out stuff. Section on adverse selection is really well-laid out. Problems at the back of chapters 13 and 14 are pretty interesting and actually can be solved through intuition alone. Weakness: Too arcane and theoretical for consumer theory. By "too arcane" etc I really mean that most professors at most schools never seem to test you on proving (for instance) the rigorous results on quasi-concavity, strict concavity and the like. Rather they like small tricky problems with Slutsky matrices and Euler equations. Yet if you read MWG you will spend a lot of time (for example) poring through proofs of the continuity of the expenditure function and representation theorems for preferences. It is true that Profs lecture and set problems out of Varian on this stuff and then rather cruelly give you MWG as the "reference text." Producer theory: this is really useless, except for some nice diagrams on cost and output. But again a good intermediate text has this too. Again most professors most places do not set the sort of problems you find at the back of MWG. Intermediate: game theory. Relevant problems and good examples, but you can get too bogged down in writing out strategy sets and correspondences and the like. But: overall my criticisms of this book are linked mainly to the discrepancy between actual teaching practises and the content of the book. As a self-contained book, MWG presents matters formally and maybe a bit tediously, but it does provide a feel for the elegance of modern, mathematical economic theory. I learned this too late for my own good, but one other advice to users of this book: abandon the math when you solve the problems. There are lots of problems in here that primarily test your ability to set up problems and your intuition.
Rating:
Summary: Ask yourself one question
Review: There appears to be a significant difference in opinions regarding the approach used by MWG. I believe that at the end of the day one has no choice but to conclude that the mathematical dependence by MWG is not conducive to critical thinking. Such a conclusion reveals the tragedy of modern day economics: mathematical elegance is preferred to practicality and intuition. The main question that a reader of this book should ask is: Does MWG present a framework of analysis that enhances an individual's ability to develop economic policy that can effectively address the economic problems we face in the real world? I believe that the answer is unambiguously NO. Instead, this book represents all that is wrong with modern day economics. That is to say that the real world plays a minimal role in MWG. In fact, most economists seem increasingly less and less concerned with solving real world problems. A look through most mainstream journals reveal that many economists enjoy inventing mathematical problems and presenting a clever mathematical solution to them. Whether what they have produced contributes to our knowledge about how the world works is rarely, if ever, a consideration. Economists need to decide whether they want to be theorists or contribute to policy. These two roles require significantly different skill sets. Unfortunately, to pursue graduate studies in economics, students are required only to learn the mathematical tools for theory; there is no requirement or real inclination in graduate schools to instill students with a firm understanding of the political economy that goes on in policy work. This explains why macroeconomics and econometrics has proved to be increasingly unhelpful, and many cases causes damage, when used in practice.
Rating: 
Summary: Advice from an ex-reader
Review: Things to know unless you are an econ student who's already done Varian and Kreps at the graduate level & are totally comfortable with that:
- you'll need to supplement this book with Varian and Kreps. Kreps provides good intuition but Varian helps you break in gently. Note that Varian alone will not do - doctoral coursework in micro is much too sophisticated. I literally went through all three books sequentially - Varian first, Kreps next & MWG last to get what was going on in MWG.
- Try to do coursework in differential equations, real analysis & linear algebra before you touch this book. At the very least you HAVE to understand differential calculus really well. The math in this book is incredbly hard otherwise (trust me, I was in misery for 2 semesters).
- This book is great for general equilibrium models etc., but the game theory stuff gets out of hand really quickly. Be sure to use Tirole (Industrial Organization) & Kreps to guide you through the game theory in this book.
To the graduate students in business that aren't majoring in econ: you won't use 80% of what this book teaches you, but if you internalize even 30% of it, you will benefit for a long while.
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