Yeah, I got Gears PC when I got my new rig a couple weeks back. At max (in DX10, which apparently only adds AA) at 1680x1050 it definitly looks better than the 360 version (textures and such), though I noticed the effects were a little toned-down. The motion blur when you roadie run around doesn't extend as far from the edges of the screen, and the fog effects seemed more transparent. But had I played it on the PC first (and if I was a dedicated PC gamer) I'd probably have preferred the PC version over the 360 one.
And yeah, COD4 is AMAZING. I paid very little attention to the series before it (post COD1, they were all the same WW2 shooter, only prettier than the last) so maybe the gameplay is still pretty much the same, but I thought the gameplay was very well rounded, it wasn't too difficult or too easy on the setting it assigned me (based on a short level before the game 'starts'), it looked pretty good... and it had my favorite level out of all the games I've ever played in my life: I won't ruin it for you, only to say that it's pretty close to the beginning, and you're in an airplane.
Never did any of the multiplayer though.