Alien Swarm is a great game, make no mistake.
The problem is that it has just one official campaign, which won’t last you any more than 1-2 hours.
Many community mapmakers have created community campaigns and maps, but sadly, the general playerbase doesn’t want to download these from an external site, for whatever reason.
It’s a shame, as some of these campaigns are really, really great.
However, some folks in the AS community have put together a massive 2010 community mappack, containing the best maps and campaigns of the entire year.