A week or two ago, I saw said adaptation.
A group of us went along, roughly corresponding to our pub trivia team, all bar one of whom had read the book. That's an important fact, because this is a film that is, more than most, coloured by what you have or haven't experienced from the book. I can only speculate - and I will - what the film might be like for those who haven't read the book.
First of all, it has to be said, the movie is generally very faithful to the book. There are some sections cut out (the thread with the kid reading the comic and the newstand owner, and the stuff about the comic book writer, are omitted, and the ending is changed), but most of the style and plot are intact. This might be a problem if you haven't read the book - the consequence is that the film is quite long, and quite broad.
The film is also quite violent. I guess this is to be expected from the director of 300, and when I think back, the book was also probably quite violent, but I still found it confronting at first experience. The performances are generally sound - performances aren't what you generally look for in a superhero movie - but Rorschach and Dreiburg deserve mention for being better than the others. The romance scenes are handled with remarkable success - eliciting the same humour and feeling that the book had.
Without giving things away, I had no problem with the changes they made to the ending. I don't quite understand why they made them, but it wasn't my favourite part of the book, and the movie ending might actually have more relevance to the rest of the movie, in terms of character arcs.
Anyway, enough rambling. Go see it - its colourful.