The more I see Brad Pitt in action the more I think he is an incredibly gifted actor. I hadn't seen 12 Monkeys in a very long time, and pretty much all I remembered was how good he was. If my memory serves me, this was one of the first roles in which he tried to diversify was he was doing, the kind of character he was playing. I think he might have been trying to prove he wasn't just a pretty boy, and he certainly succeeded. From here he would go on to play a wide variety of characters, with some (to me, at least) memorable turns in the more recent Mr. & Mrs. Smith, Burn After Reading, and Inglourious Basterds, proving his versatility and fearlessness in choosing parts in a movie.
As for the movie itself, I really like it, but then again I am partial to time-traveling stories and time-continuum paradoxes. Plus, "The Army of the Twelve Monkeys" has to be one of the coolest names for an army ever. Or one of the strangest. Or both.