I've Got The News Blues wrote on Feb 12, 2010, 12:13:
Asking for this hurts PC gaming. While I would prefer that PC games support as many types of peripherals as possible, I don't like UI's, HUD's, and aiming which is designed to accomodate the limitations of a gamepad unless the game is best played with a gamepad such as an action sports title or a vehicular combat game. Since development resources are usually limited, PC gamers can't have it both ways. We can't get titles designed to accomodate the extended functionality of a PC and also expect a simplified console game for the relative few who want that. So, given the choice between a quick-and-dirty console port designed for a gamepad and a game designed for a PC's keyboard and mouse, I would prefer the latter.

So, if you want to play a PC FPS game with a gamepad, use one of the keyboard and mouse emulation utilities that come with some gamepads or a third-party one like Xpadder.