I'd buy a console tomorrow if they allowed mouse/keyboard - sick of all the cheaters in PC gaming, and having games get _better_ as the hardware ages would be a welcome change from having to constantly upgrade my video card to play new releases at a decent frame rate!
[1] http://xim3.com/
The way FPSs interpret mouse movement is by mapping units of mouse movement onto units of rotational movement. If you move the mouse x units to the left, your view will rotate f(x) units (an absolute relation). When mapping joystick movement, the location of the joystick determines the speed at which your view rotates. If the joystick is x units off center, you will move at f(x) units per second until the joystick moves again (a relative relation).
For this reason, it's pretty simple to convert relative joystick movement into mouse movement, since you simply need to map the joystick position onto an absolute mouse velocity. However, it's extremely difficult to turn the absolute mouse movement into a relative joystick position. From what I can tell, it takes the velocity of the mouse movement and returns it as a joystick position, so if you're moving the mouse at x units per second, it will return a joystick position of g(x), which the game will then turn into f(g(x)) units of rotation per second.
What you end up with is jerky and unresponsive. Mouse movement just can't be translated into joystick position in a transparent way. I used the thing for a couple days, and ultimately gave up on it. Even as a hard core Mouse/Keyboard gamer, I preferred the controller over the XIM.