Well, as someone who has only played FPSs, I'm loving it. I'm sure someone with a far more varied gaming background can comment on it's ability to do both...but it is a ton of fun to play MP.
Basically, you and your friends kill a bunch of things, steal stuff, accomplish missions, shooting more crap, running acid-spitting warthog things down with majestically maneuverable rocketcars whilst fleeing giant raptor bird things that poop on gigantic windmills (that's also a side mission) and moving between different levels that all look vaguely like the Southwest and a tf2-esque building style.