But the main reason for me is the controller: It's just so much better than any other controller I've connected to a PC. The back triggers have excellent force feedback and in some games the vibrations tell you on what type of surface you're walking. This might not count for XBox users, which can use the controller on their PC.
Also, I use a Mac, and I don't want to bother with setting up a gaming only Windows PC. If I factor in time spent, $500 is worth it.