In my opinion:
Mouse&Keyboard are still the most precise and efficient way to control FPS, it works perfectly.
Wiimote is not more realistic, matter of fact it focuses on the sensor bar it's not like 1:1 aiming on screen (I remeber the G-con), but on the other hand I find it more fun to use, it gives you the feel of shooting, also it's still a way more accurate than the dual analog system (Metroid Prime is a sufficient proof) while it's as ergonomic as normal gamepad (and not like a keyboard) plus it doesn't constrain you to play on a desk. Using the wiimote patches for example I could play PC fps's on a 32" TV insted of on a 17" monitor.
Finally, it won't work well only on FPSs but on any action game involving shooting like Gears Of War,Uncharted,Lost Planet,Dead Space ( a real one not like extraction)
