I enjoyed it, but it was rather unnecessary. Silk Spectre I & II's actresses were awful, but Rorschach, Comedian, Nite Owl II and Doctor Manhattan were spot on. The visual effects and the action scenes were great and even Zack Snyder's slow-mo shit that I hate in every other film seemed to work in Watchmen. I see why they changed the ending, but it creates a few plot holes. I also had a problem with the prison fight scene, the film had a great soundtrack consisting entirely of songs from the '60s to the '80s, but for that scene they just used a generic Rock-Techno track that seemed so out of place with the rest.
The main gripe I have about it is it seems to just be a story about the "Watchmen" (that really irks me that the group actually calls themselves the Watchmen, considering the word can never be seen in full for the entire book, bar the covers), rather than what it should be: A deconstruction of the superhero genre, which Zack Snyder didn't seem to get, and just turned it into a normal superhero action flick.
That said, the opening credits brought a tear to my eye since Watchmen is my all time favourite comic (I'm reading through it again right now), it was absolutely beautiful.