I really enjoyed Watchmen. The book is really dense, and although it's ultimately good it was very hard to get through the first time but the movie did a good job of adapting it. Unfortunately the fanboys, I guess like most fans, don't realise that you have to trim these things down for the screen.
I've read The League of Extraordinary Gentleman, V for Vendetta, Watchmen and From Hell and League aside they all were good adaptations if you aren't too precious about the source material.