I really loved Halo on X-box, but I didnt appreciate it enough until I played it on PC. I have to admit it was all about the controls for me. The X-box controller feels like a loaf of bread with buttons and joysticks on it!
But, Im biased, I had been playing CS and MoH for a long time before I ever played Halo - so perhaps it was simply because I was used to it. However, ive been playing Halo 2 with my friends for a day now, and man...its great. I dont think it wins out over CS:Source for my dollars, but its damn good, and it will be rounding out my Christmas treasure.

P.S. Metroid Prime 2 is a no brainer for me, I just have to wait until i get my next check. Im definitely a GC fanboy, the next 6 months are looking amazing for us (GC owners)!!