I guess every single man who has a Steam account plays it, cos' it's a fully free game from a great company. It's a great game with all those Steam-features such as Steam achievments, Global stats, matchmaking and so on.
Me and my friends love it and we play it almost every weekend.
There are new campaigns, they're simply unofficial. If you visit Steam's forums you can download community made campaigns, then you just extract them to the Alien Swarm directory and away you go.
For a free game it's extremely impressive. Anything made by Unreal Tournament 2004 developers and managed by Valve would be, but this game has an addicting side to it what with all the Steam achievements integrated into the game. I just wish people were a bit more social and thoughtful about the game, and would devise tactics on the map and talk to eachother more often. In terms of gameplay, it's spot-on.
If you want to add me and play it some time my Steam account name is Kervenv.
