Yeah, I once was in a really amazing game in Uncharted 2's version of "hoard mode" (3 humans vs. waves of increasingly nasty CPU enemies) with 2 other random players. We made it all the way to the last wave, which was all armored dudes. One of my "allies" accidentally took me out with a grenade while I was trying to reach a rocket launcher (you cannot be revived from an explosive death). The CPUs took him out soon after, so it was just the last player versus 5 armored CPUs. The guy then systematically took out all 5 over the course of the next few minutes. It was pretty awe-inspiring.
I've seen really great teams come together through random matchmaking, and I've seen terrible teams (mostly in games that place emphasis on individual progress like Killzone 3). The thing is, I don't play these games enough to want to be shackled to clans. It's like my best friend with WoW, where he's compelled to play WoW twice a week at specific times because that's when his clan is on.
And yeah, voice chat is both the best and worst thing to ever happen to online gaming. I was playing Killzone 3 in Objective mode the other day trying to finally win a game as a member of the defensive team. While the rest of my "team" was running to engage the opposing team, I bunkered down by one of the objectives and started placing defensive turrets. The enemy team would keep respawning, and all they needed was one moment of weakness at each of our objectives and they'd win. We ended up winning, and someone on my team with a Mic called me a "bitch" for actually defending an objective rather than doing glorified team deathmatch. And that's one of the kinder things I've heard in online chat.