I think they could have pulled off the original ending.
Other than that, it was pretty spot on. The Silk Spectre II was kind of wooden though. Rorschach was the best, great casting as well. Besides the Silk Spectre II, it was spot on. It dragged a little, but I think I agree with Alan Moore that it was better when I was able to read it at my leisure in a comfy chair by the fire.
Still, it's a great flick. 4/5 for me. Read the comic, and thought it was a good adaptation. I don't think it says much of the director that he can imitate comic frames, but you gotta appreciate his respect for the source.
Oh yeah... what was the song playing during Doc Manhattan's arrival on mars/montage?