Finally got to play some multiplayer with a coworker at lunch. FUNNN.
Our favorite mode turned out to be the "Catch the Stash" one where you try to be the first to get to the van full of money and drive it to a goal. They give you Proximity Mines from the get-go, so you can drop timed mines behind you, Spy Hunter-style, if the other guy is on your tail.
The "Defend your Base" game is great, too, but if you're not a little organized, you will die quickly. They attack in pretty fast waves (usually a car careens into the area like that guy at the start of The Stand, and tries to run you over, then four guys jump out and open fire on your and your partner.) Find a good weapon and armor fast. You get grenades from the start in this mode. Very handy if you can time one to blow before they get out of their vehicle.
Racing is what you'd expect. There are two other fighting modes that are interesting, but that weren't our faves.