I totally am stealing this from a recent Steep and Cheep email - I admit it, but it was too good and true not to pass along.
"If you don't like a book someone recommended, keep it to yourself. They won't take it well when you tell them it was bad, and they'll be especially upset if you launch into a well-reasoned explanation supported with literary precedent. I learned this not when I compared the Poisonwood Bible to a doorstop, but when the friend who had recommended the Poisonwood Bible responded with a dismissive shrug when I asked what he thought of a book I'd loaned him. Books take so much of an investment in time that I think they form a bit of a cognitive dissonance for their readers. Unless you quit reading halfway through, it's like you've invested so much time in the book that it had better be good. Taking a particular liking to a book is similar to finding a song you enjoy deeply--there's something about it that resonates with you. So when someone comes back with a negative review of a book you like, it's more like it informs your differences with the person than your taste in books. And that person can never be trusted again."
Proving yet again, as said in High Fidelity, it is what you like, not what you are like.