Bad open letter aside, I'm going to say something that might upset some people.
First off, I loved the book. I read it before I saw the movie (like, before I knew there was a movie coming out, two or three years ago), but I'm going to come out and say this:
I thought the movie had a better ending than the book.
Unless there's something that I'm missing (someone please point it out to me if I am), Moore's ending was just overly complicated, without adding anything significant. *kind of spoiler* blaming Dr. Manhattan makes more sense, and is simpler.
The fact that the movie basically stuck to the comic book word by word, shot by shot, might make it a bad original movie, but you definitely can't say that it wasn't true to the watchmen. Also, I thought that the opening credits were really well done.