DoD Source is pretty much the only Steam game I play... I own CS:S, TF2, CoD4, INS but hardly play any of them. I enjoy the WW2 theme and the overall gameplay.
If/when you get it, I'd say check out all the maps, and decide which is most fun to play, then join that house.
For me, flash is the cat's meow. So I joined Hell Hounds. If you like flash you're not limited to the Hounds, though. Reservoir Dogs shares flash with us.
It's truly worth it to shop around and decide which house is best for you. Checking their forums helps too.