A mouse is easier to point and shoot with. This is practically a universal acceptance. It's the reason why some online console games like UTIII will have a stipulation for matchmaking of "mouse/keyboard" or "no mouse/keyboard allowed."
There's no steady aim involved since the crosshairs only move when the mouse moves, which is resting on a stationary desk. It is the easiest point and shoot control scheme available.
The only pointing device that trumps a mouse is a pen and tablet. Yes, more accurate than a mouse which is why most freehand professional graphics are done with a pen and tablet since drawing with a mouse is the equivalent of drawing with a bar of soap.
Learning the keys for the typical FPS game is the same as learning or assigning keys on a 12 button game pad. Most games require the user to learn the layout if they don't provide unlimited key assignment.
The only extra intelligence required in PC gaming lies in the ability to build, modify, optimize and maintain your own rig.
(If you can do all that successfully, it's more fun than playing games on a PC IMO.)
