I'm a fan of FPS games (on the PC, not so much on console) but I've never really enjoyed squad-based games because it's usually kinda annyong to control your AI team-mates. So while I happily played all the way through Far Cry, I've never had the patience for stuff like Ghost Recon.
I messed around with online shooters for a while but never really got good enough to enjoy them, mainly because everyone else is way too good, and it's hard to get any kind of teamwork going when you're playing with people you don't know. The one I did get a kick out of for a while was Day of Defeat, the Counterstrike mod, because even with the problems I just mentioned I felt that you were almost forced into playing as a team and some of the pitched battles could get really immersive.
Having said that, I checked out the demo video for Brothers in Arms, the new WWII shooter, this morning, and I have to say it looks pretty damn exciting. The team AI looks to be great, and it looks like it might fix some of the problems I've had with squad-based games in the past, such as situational awareness. If you're into this sort of thing and haven't seen it yet, take a look:
http://www.gamespot.com/pc/action/brothersinarms/media.html?gcst=bia_ost0302.asxClick on the "Brothers in Arms Demonstration" video. It's about 15 minutes long but well worth it. Anyone else looking forward to this game?