Pro's
The opening half of the movie...in which New York is vividly recast as post-apocalyptic dystopia.
Smith's portrayal of a man slowly going insane. Considering this is a big-budget Hollywood movie, this is subtle and quietly moving. Flashbacks to pre-apocalypse confusion are interspersed with care and inform Smith's character and the plot.
The idea that it is Smith who is now the monster. After all, from the perspective of the other residents of the new New York it is Smith who is the outsider. He lurks in his basement like a Frankenstein conducting lethal experiments on innocent subjects he captures.
The sense of dread. This is built slowly and cumulatively. The scene where the dog runs into a dark, seemingly empty building is exquisitely tense.
Sam the Dog. Seems strange to praise a dog for its acting, but for most of the film it's just Smith and Sam and they make a fine double act.
Con's
The second half of the movie. Once the monsters are revealed in all their CGI glory the movie turns into a loud, shouty action flick. The psychological and moral arguments are shunted out of the way and we are left with a series of ever-louder explosions.
I'd consider this an interesting but flawed movie. The first half is great - filled with loads of striking imagery and interesting thoughts about isolation, survival, personal morality, the boundaries of sanity and human arrogance. The second half is more interested in bombs, guns, and monsters. Worth watching for the performances of Smith and Sam.