I bought and watched both parts tonight and really enjoyed it as a whole.
I thought it did a very good job at being both faithful to the original story while streamlining the narrative to suit the flow of a film. The animation was superb, especially during the fight scenes. I was very surprised by how well done the fight "choreography" was. Some real hard-hitting stuff. The fight scenes in the films are more protracted than in the comic, but despite this the pacing overall felt pretty good. Well, Part 2 felt a bit more rushed than Part 1, but that's probably because a lot more happens in quick succession in the second half of part 2.
I do wish they had found a way to integrate more of Miller's first-person internal monologues for Batman into the film. A lot of that stuff made the book what it was for me. Also, I feel that some lines of dialogue could have been better if they had more closely adhered to Miller's text -- for example, Bruce's last words to Clark at the end of their battle involved berating the latter about him becoming a stooge and a joke. This was taken out of the film and replaced with something about Bruce wanting Clarke to experience mortality like the rest of us, as if Clark has never had encounters with green rock or death before? Didn't sit well with me.
Some of the subplots were excised, which I'm not sure how I feel about. The interviews with citizens of Gotham being cut out was probably a good decision, but there was this one really powerful scene in the book - involving a mother who is killed attempting to bring home a gift for her son - that I wished had made it into the film, if only to underscore how brutal the mutant gang was. That was the aspect of the film that I felt was undermined by the paring down of scenes in order to keep the run time in check. However, The Mutants don't come off quite as menacing as a result. Still, the film does a better job of compromising than most other DC animated films.
The voice acting was generally on point...except in Batman's case. I don't want to be too hard on Peter Weller's acting here, because I think his voice sounds like a natural fit for an elder Batman speaking at a conversational volume. The problem though is that during moments when Batman expresses agitation, emotion, or anger, Weller's performance falls flat. I let it pass for the first half of the film, but as the movie progressed and the escalation of intensity in the plot developments required more emotion to be expressed by Batman, Weller kinda let me down. It was specifically Batman's speech given on horseback to the Sons of Batman army that bummed me out the most. That should have been a powerful moment, but it was compromised by Weller.
On the other hand, I loved Joker's voice. It was understated and almost serene, making this particular portrayal of the character that much more disturbing. As for everyone else, the main cast sounded how I expected them to, which is good.
I'm generally happy with how this turned out. It may be my new favorite of DC's animated films.