The movie is very good visually. It's dynamic.
Most of the acting is pretty good, Silk Spectre II being the glaring exception. If you didn't expect the blue wang, you likely didn't read or pay attention to the novel. Dr. Manhattan's nudity is an important part of his character. I won't say it wasn't a bit distracting, but it has to be there. I think they could have made it a little less "out there", but it had to out there to a degree. I was a bit disappointed in the apparent need of Snyder to ratchet the violence and brutality up a bit from the original story, as well. The tone is already pretty dark and some of the movie violence seemed gratuitous.
THIS IS NOT A MOVIE FOR KIDS.
There was far too much detail left out. There's nothing "extra" in Moore's story. Everything is there for a reason and the movie suffers from a lot of this detail being left out. I thought the new ending was better for a movie than the original, but I felt it brought the viewer to a different conclusion. That's open to interpretation, though.
I think they should have just made this a 4 or 5 hour film and included more of the necessary details. They could have broken it up like the Lord of the Rings series and told the whole story pretty easily in 2 parts. Otherwise, a 12 part, 12 hour miniseries on HBO or something that followed each chapter would have been very cool.
As an accompaniment to the graphic novel, it is pretty good, I'd give it a B+. As a stand alone product, I'd say it had some holes storywise that really made it suffer, I'd give it a C for story, and a B+ for the other aspects of the film.