I'm not into online PvP FPS. I can't move that fast. Constantly running = No Fun. I hated Quake 3.
(I really got my jollies from PC Quake 2 and Nintendo Quake tho)
My ideal FPS is single player with monsters you can stalk. I like to chose my position, aim, and take them out clean. Of course there is the excitement of the monsters that surprise you - that's fun too.
I like to have the whole world in my hands - a gamepad with all the buttons I need. Trying to find - and remember - buttons on the keyboard and move the mouse around at once.... it just aint confortable.
I know you can aim faster and more accurately with a mouse. I don't care. I don't want to have to aim.