I saw this last night.
I read the comic a month or so ago so was quite looking forward to it. The film was as good as it could have been. There was no way they could have tweaked it to make it better. It was long, but it couldn't have cut more out. The one criticism I had was the soundtrack which was generally exellent, but it had cheesy electric guitar music in places which didn't really fit. To be honest, that is being overly pedantic.
The acting was acceptable, the violence was retained (in fact, it was exaggerated). What stopped the film being perfect for me wasn't anything except for the fact that it was a film.
What made the comic so great was the build up of a world (like LOTR). Journal entries, newspaper snippets, commentaries etc at the end of chapters really immersed you into the atmosphere - what it was like being a superhero. That just isn't possible to do in the medium of film. The story of Watchmen requires lots of backstory, which makes it quite slow. I could imagine the length of the film might be too much to please some.
Also, I would like to state that the new ending isn't better or worse, it's just different and more suited to film. The dilemma is the same.
A very good film.
4/5