I've been playing the Call of Duty 4 multiplayer beta.
I should say trying to play it, as it's been pretty much an internal grudge match to do it.
Gameplay and mechanics are pretty solid, I really like the skills and rank progression, and the ability to customize my loadouts is nice.
But the beta doesn't at all seem to care about Xbox Live mechanics like zones, or preferred players, or players to avoid. To be honest, these games have been some of the worst I've ever experienced crowded with foul-mouthed, racist, immature putzes.
I'm a pretty tolerant guy, but this has been so bad that most of my games start with me getting waxed as I'm trying to mute the worst people, then I play pretty well, then I leave the lobby to do a complaint or two -- just about those folks that egregiously violate the XBLA UA (like those vociferous, vitriolic racists; that's not something I think people should play with).
On the upside, I've met a couple of nice folks, interestingly all from outside the U.S. (but currently stateside) -- guys from New Zealand and the U.K. Nice blokes, and less gutterish than my in-country brethren. Sad.