outside of novelty and "Because-I-Can" value, i can't see how Kinect would be a viable input method for FPS's in it's current form.
trying to remember which gesture does what and blowing off your characters own foot because you scratched your nose accidentally might get a bit tiresome.
For years people have been coming up with alternative controllers for FPS's, especially on the PC, but nothing has come as close in popularity as the good old mouse and keyboard.
Also, i have a suspicion that anyone trying this style of input method for an online FPS against others with controllers is in for a rough time.
Kinect is awesome, but i don't see the point in trying to adapt it to a situation like this where it not only might not shine, but could very easily put people off if not executed properly - much better imo to use the Kinect in more innovative genres.
Devs would have to come up with a shooting FPS-like genre built from the ground up for Kinect to take many scalps in this arena imo.