I saw Watchmen on Thursday morning, in iMAX Manchester. This was a special screening for UCI employees, of which I am one. Since then I've been trying to think about what I can say about the film very carefully. It was really tough, so much to the point that I considered not writing anything, but I couldn't leave my avid readers - of which I'm sure there must be at least three - waiting.
In the end an alternative has been offered to me. I just read a fairly extensive review of the film by, David Chen, posted on the slashfilm website. I almost entirely agree with what he has said and cannot truly elaborate, or say anything without heavily plagiarising his work. So instead I offer you a link to his review. http://www.slashfilm.com/2009/03/06/movie-review-watchmen-a-cinematic-achievement-in-adaptation/ .
I will say one thing his rating is maybe a little low I think I'd raise it by .5 or 1.0, but I'm not entirely sure ratings are a sensible thing for films, see my post on the subjective nature of film viewing for more info. There are some spoilers I think if you haven't read the book, but I both suspect and hope that those who want to see the film have read the book.