Fired it up, playing the multiplayer. Holy. Shit. This is fun.
The Scientists/Jet is the Medic/Engineer/Disguises class. Their jet mode has lock on missiles, meaning Air to air battles between Scientists is insane to watch and participate in.
The soldier class is the all around beat-your-ass class. Their chaingun is extremely powerful, as is their whirlwind melee special. The real powerhouse on the battlefield.
The scout class.. I haven't played a whole lot. Mostly moving around like a maniac and killing people before they know you're there. They have perks to do extra damage when uncloaking, and extra damage when they attack from behind. They also get access to shotguns.
The leader class is something of a support/front line attacker. They're made to push the front line forward and draw fire - they have a shield ability. They also have a frenzy sort of thing that buffs armor/damage output.
There's 25 levels (I think) Per class, and after you hit 25 on all you unlock something called Prime mode. I'm not sure what that does just yet. Probably like prestige.