[Spoilers, don't really wanna black out everything...]
I too thought it was fairly ambiguous as to who Brad Pitt was supposed to be. They talk about his time in past wars, but not really what he did. I found it odd they sent a few Navy SEALs with him but they die almost immediately and he makes it through all of that.
It seemed to me Brad Pitt brought death everywhere he went. Finds that mexican family, it implies they all get killed. Goes to S. Korea, gets their captain killed, the SEALs killed, the doctor guy dies. Goes to Jerusalem which has been doing great all along, it falls the day he gets there. Boards that plane, it gets overrun by zombies so he blows it up with a grenade and none of the other passengers live.
I didn't care much for his family storyline, I really didn't care about them at all and figured they wouldn't die or anything. His wife was also annoying I thought, always crying on the phone or whatever. She's also weird looking, especially when she smiles - not her fault, but kind of distracting.
Also didn't like the typical Hollywood magic where he goes through all this hell on his journey, but when he decides to go back home he just does so with no mention of the return trip. I also would rather have actually learned about the origin like it was originally set up for.
Now I know it seems like I shit all over the movie, but I really did like it. I'd give it like a 7/10. It was an intense movie with good special effects, I was never bored and it had some unique aspects to it. I just think they had so much more in the story already that they could have utilized better but instead focused on things that weren't as interesting - like how that nuclear bomb went off and we never find out where or why that happened.