Let me preface this with the following: I have in fact read the graphic novel, and I believe that contributed heavily to my understanding and enjoyment of the film.
Ok, now that that's out of the way...
I loved the movie. It currently stands as one of my most favorite films (up there with Terminator 2, Juno, Iron Man, and the Godfather Parts 1 and 2). The Watchmen is a culmination of everything I look for in a movie: Solid performances from multiple cast members (not everyone played their parts equally well, but there was certainly no Sofia Coppola ala Godfather Part 3), gripping/well-thought-out story line, and a perfect soundtrack...perfect in that it could not have been any better in my opinion. The music fit the irony of the entire story. Perfect.
That all being said, as I mentioned up top, I do think that in order to get a solid appreciation for the brilliance of the movie, you really really need to read the graphic novel ahead of time. The "characters" in the background (*wink*), the excellent and wise change to the ending, and the overall feel for the world of the Watchmen have to first be experienced in their original form. I had a ton of friends go into the theater expecting Spiderman even after I explained to them over many in depth conversations the significant depth and lack of "Hollywood epicness" of the graphic novel.
So to summarize: Unbelievably brilliant movie...read it first.