Yeah, a lot of my friends said that this movie was horrible, and that the last 30 minutes was the only good part to a boring movie, but I felt completely opposite. The visuals and exposition in the first three-quarters of the movie felt extremely powerful and satisfying. Add that to the fact that whilenot only is the ending is completely different from the book, it doesn't really work in the movie either, it feels tacked on, as if they tested the other ending and audiences didn't like it, so they went back and did a re-shoot, leaving us with this crappy cop-out at the end.
I'm agreeing with others that the woman was definitely the weak link here too, her lines felt rushed and unconvincing, and it felt like the director tried to make the audience shift gears and empathize with her character as soon as she got there even though we had spent the entire movie so far focused on Will Smith and getting into his mind.
Also, the CGI infected weren't completely sold on me, but even with all these little things this is still one of the best movies I've seen this year, and also second on the biggest let-down endings, right behind No Country For Old Men, another great film I've seen this year.