I thought it was a reasonably well done movie, having more in common with Contagion or Outbreak than The Walking Dead. I didn't read the book, so that was probably for the best given it seemed to have nothing to do with it. As much as I love TWD, I've been getting a bit tired of the zombie genre's phase of being all survival and character-oriented instead of having a "hero" or someone to root for. Even though the ending is kind of ridiculous, at least it had a twist or two I hadn't seen before so that was nice. I'm not much of a Brad Pitt fan but I thought he did a reasonable job carrying the plot.
Interestingly, the original 30-40 final minutes or so was completely different from this version. That version actually sounded like it had a little more of the edge from the book, and it seemed interesting but definitely much more bleak and depressing. Here's some info (caution the link has plenty of movie spoilers):