It's pretty dangblasted good. Up there among my favorites of the animated superhero movies (as a gauge, the others on that list would be Under the Red Hood, Mask of the Phantasm, Planet Hulk, and Crisis On Two Earths). It's very nice visually - it's got a more traditional color pallette than the book, but it's still appropriately moody, and the animation is about on par with the Year One movie. The action sequences in particular were very well executed.
It's not as good as the book but it was never going to be. A lot of it is a little too faithful to the book narratively, which gives the story a slightly disjointed, lurchy feel. It feels like they just animated what was on the page without going the extra mile to make sure it all came together as a fully-rounded movie (I had similar issues with All-Star Superman and Watchmen).
Very minor spoilers about how it differs from the book:
Spoilers: (Highlight to reveal)
It gets rid of most of the patented Frank Miller inner monologues. I think this was a mistake, because in the book they helped a lot in illustrating Bruce's grim determination, and it would've been nice to hear more of Robocop's Batman voice because he did quite a good job.
But anyway, yeah, overall very good. I'd recommend waiting until Part 2 comes out before you make a purchase, because they'll probably put out a sweet double disc pack, and I suspect it'd be more satisfying to watch both parts in one sitting so you get the complete story.
