Do you read reviews (at actual review sources--like Publishers Weekly or School Library Journal, or those posted by fellow readers at Amazon or GoodReads--not just a friend talking about a book in their blog, which I'd consider more word-of-mouth than review) when you're deciding whether or not to pick up a book? How much do they influence your decision? What sorts of things do you look for in the reviews?
I quite frequently consult Amazon and/or GoodReads when I hear about a book from a source I'm not completely sure has the same tastes as me. (This is my way of keeping my to-read list of getting completely out of control. If I added every book I heard anyone say something good about to it, it would take me a year just to scroll through it!) More often Amazon, as they usually have "official" reviews posted along with the reader reviews.
When I'm reading the reviews, I look for two things. Mostly, I skim the good stuff and look for the criticisms. People tend to be much more specific about what they didn't like about a book than what they did. Every book has flaws. Does this book have one that's a particular peeve of mine, or makes it likely that I won't be able to get into it? Or are they things I don't really mind, or may actually see as a point in the book's favor? Everyone has different tastes, and I know I'm much more likely to enjoy a book that people say has amazing characters but a slow plot than a book with cardboard characters and a plot that just zips along.
I also sometimes have some idea that I might not like a book, based on the book description or other comments I've heard about it, for one reason or another. Maybe it sounds derivative of other books I've read, for example. When I'm skimming the reviews, I also look for indications that these potential problems are not actually there--e.g., if people are saying what a unique take it is on the idea.
I don't give every review the same weight. I trust the professional reviewers more than the average reader (though not completely). I trust a well-articulated and thoughtful review much more than a hasty, grammatically-challenged one. And it's also about numbers--the more often I see the same complaint, the more likely I am to believe it's a real problem and not just a matter of someone's taste.
And like I said, this is assuming I've only heard about the book otherwise from people whose tastes I don't know that well. If several trusted friends say great things about a book, I don't even bother checking reviews.
How about you?