The movie was great! As a comic-book movie it wasn't as good as The Dark Knight but few movies are that good.
It followed the book remarkably well, in fact, the ending was cleaned up to be better than the book.
As I was watching it I was wondering if someone who hadn't read the book would be lost. I asked people afterwards, some said yes, some said no.
My one criticism would be that the movie didn't show enough of how early comic-book superheroes were so "truth, justice and the american way" type people. When the book came out, the reading audience knew about this status-quo and Moore didn't really need to delve into that. Few books before Watchmen showed them as having different motives or downright crazy.
What got Alan Moore all jaded about his work being turned into movies was From Hell and The League of Extraordinary Gentlemen. He took the money from those and the studios went and bastardized them. He hated the thought of both V for Vendetta and the Watchmen being made into movies based on earlier experiences and his dealings with DC. He really should have no complaint since they both turned out so phemonenally well.