I actually pirated a copy to see how well it would run in a VM since rebooting to PC is always a pain. It ran well enough in a pinch, but I would reboot to play it if I could at the time. At that point I figured why the hell not, it's plenty fun and brought it. I then threw the pirate files into where Steam would install it, saving me a large (re)install and a whole gig since the pirate version was very well compressed.
So yeah, pirate it, if you like it enough, go buy it. If you do get it for Steam, feel free to look me up for a game. Even when playing from the other side of the world, it's very playable since there isn't any "Verses". Although VS wouldn't really work... Some of the specials under control of a human would single handedly demolish a team since they don't have limitations like they do in L4D.