Actually, the gamepad really works well for this game. I've never played an FPS on PC with a controller, but I gave it a shot with Halo. It works very well.
Search "xbox usb" on ebay. I got one for $7.00 and now use my controller S on my PC. The upshot of doing this is that you don't ruin the controller for the xbox (no offense, Rent) and it comes with drivers that accurately map all the pad buttons (including the depressable analog nobs). The result is gameplay exactly like the Xbox (eg, perfect for a console) with better graphics and much better multiplayer.