I haven't, but from all the trailers and gameplay videos I've downloaded and watched, I can say that Gears of War is easily the Xbox 360's first killer app.
If you gave me a choice between the PS3's Resistance: Fall of Man and the Xbox 360's Gears of War, I would choose Gears of War with out hesitating. The gameplay is just so innovative and FUN. Using the chainsaw bayonet on the COG's Lancer Assault Rifle is easily one of the most gruesome and satisfying kills if you manage to pull it off. It's one of those kills that'll make you stand up and cheer.
I'm not saying that Resistance is a bad game, but it's pretty much a polished FPS with really good graphics. There are some pretty neat weapons like that nail/spike grenade, but from what I've seen so far, I'm not as impressed as I was by Gears.