Just got back from City of Ember, and while there were a lot of changes from the book, I could see the reasoning for most of them. After seeing the film and reflecting back on the book, if it had been a straight adaption, it would have been boring cinematically.
PERFECT CASTING on everyone's part. Doon is just weird-looking enough without being ugly; Lina was rad (much better than creepy ass Atonement) and Bill Murray as the Mayor? I honestly can't think of anyone better.
Also, I leaned over to Tory with the opening shot of Ember and I said: Oh my god. That's exactly what I saw. The set design was unbelievably faithful to book. And the director and creative crew maintained DuPrau's atmosphere brilliantly.
Only one addition really bothered me because I felt it took away from the kids' ingenuity. But even that didn't dissuade me from enjoying this movie.
GO SEE IT!
Oh, and Tory and I did actually jump at one point. So hooray!
Edit: Good review/article alert!
http://io9.com/5061881/ember-proves-the-only-grown+up-movies-are-aimed-at-kids