I saw MW2 played at a LAN at uni a couple of weeks ago and I have to say I was unimpressed.
The lobby and matchmaking service is pretty nice, allowing you to form a party and then look for games where you can play on the same team, a pity it doesn't work so well in practise. You're stuck with the problem that a large proportion of people leave as soon as they see several people bearing the same tag join the game meaning that you have to go through the matchmaking process several times. Then there's the more technical problem of the game sometimes leaving some players behind when the lobby joins or leaves a game.
When you actually get into a game, there's a small chance that the dynamically chosen "best" host will be terrible and have huge lag spikes. (This while we're all connected through the university JANET connection.) Eventually the game decides to migrate the host, everything stops, a new host is chosen and then the game continues. Then the host quits because his team is losing and we go the the "Choosing a new host" screen again, and again.
This must be some definition of "works really well" that I was previously unaware of.