I just picked it up last night
it's basically a physics sandbox for the Source engine. the workshop has mad user created stuff, like models, maps, gamemodes, etc
it's the kind of thing that has infinite potential, but you need to think of an idea or something you want to do first. I saw the mattress and went from there...
I tried a multiplayer server, and it was a bunch of dudes working on this crazy machine. I see they also have a bunch of other more traditional gamemodes, but I haven't really checked them out yet