I picked up (downloaded) Alien Swarm and couldn’t put it down for three hours. The concept and gameplay are right up my alley.
I took a look at my achievements before I quit and got one for completing two missions online. My reward-and yet another reason that Steam (and Valve) kicks ass-is a parasite hat (heals you while draining the enemy) in Team Fortress 2!
There are many reasons this game is so addictive. The most important (I believe) is the amount of devotion gamers have to playing is matched (or more likely surpassed) by the devotion developers have to delivering.
I haven’t paid much attention to these types of games until I saw the drums at a friends place. This game literally rocks! Love those drums! I stunk when I started (and I still stink), but got the hang of it later. You really feel like you’re playing the song.