I have this discussion entirely amiably with my friend, who describes himself as part of Yahtzee's PC-Gaming Master-Race. As an inbred console mule-child I'll likely never get far into PC gaming beyond my favourite RTSs and other cursor games. Though I still love memories of the massive Unreal Tourny LAN parties in my school's computer lab.
Mouse is definitely a superior input for aiming. It still doesn't feel very natural to me using my whole arm in that way, and I think a better control is possible, just not likely so long as every computer uses a mouse interface for general work. The keyboard, however, is intolerable. I cannot play active shooters to maximum capacity using an interface designed for word processing and spreadsheets. I'll always prefer a game pad, even for PC gaming.
If we could have half a game pad in one hand (I think they're called Joysticks? From the Palaeocene, I believe) and a mouse in the other, that would be great. Except a mouse can't really be used on the couch in front of your TV. Find an ergonomic solution to that problem and we're golden.
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