My friends have been going nuts about League of Legends.
Me, I want to wait until I get Windows 7 and reformat my computer before I start playing. >.>
It is really a good game (and windows 7 rocks).
A match takes between 40 mins to an hour, usually.
If you don't have a tank, you have already lost the match.
If one of your team disconnects and the game turns into a 4 vs 5, or 2 vs 3, you have already lost.
If a few of your team mates die 3-5 times in the first 15 mins, you have already lost (its called 'feeding', where opponents get a ton of money from the kills, and will have gear vastly above yours very quickly, steam rolling you).
But it got a ton of neat characters to play, and with a good, balanced team its hella fun.