Seriously though, Halo is just plain fun. I recently went back and played a little CE. Man I forgot what made that game so good. I recommend playing some CE if it's been awhile for you Halo fans.
Theater mode is beast too. I like going back and watching a whole game from different perspectives. It's crazy how much shit you miss during the match.
Hey Sac, you get these vids from the Bungie site? I was on there a couple of days ago and saw some videos.

They mentioned the Halo Fails #5 and I went to YouTube to see the rest.  If you like videos I recommend you look up some of the sticky montages, they are hilarious.  Also, you should check out the ESPN MLG highlight ones.  Those are pretty awesome as well.  Good to see Halo 3 getting some love in this thread, most of the time hate is spewed quite frequently about the game.

After watching those videos, yes, I recall exactly why Halo multiplayer is and always has been so popular. It's the best arcade fps online experience.