I'm a PC gamer as well and I prefer mouse/keyboard controls over gamepads for FPSs any day, so I have little interest in Halo.
When I got my Xbox (not the 360) I skipped Halo. I played it in the office once and while it was fun it wasn't all that and a bag of chips.
When I buy a 360 I still wouldn't get Halo 3. I'll just play whatever is the hot FPS of the day for the PC and use the 360 for other games that I cannot play on the PC.
I am sure there are many people that are like me in this regard.