Just played ~30 minutes with a fellow Bay12er.
For free, it's damn fun. It takes a good amount of teamwork. Death is permanent in each level, so if you die early that's it until the group finishes. If you don't stay moving, the enemies start trickling in and don't stop until you make some distance. They attack in all sorts of dynamic ways too. Guys fly in from off screen above you and attack, leap in off screen, climb walls and other stuff. Just when you think you've got a good line with your firing arcs and your sentry guns, guys come in a way you didn't expect. It's all pretty well done, and has the same kind of tension I got out of L4D.
The unlocks are pretty decent too, standard. You get your choice of primary, secondary and tertiary weapons. You can carry 2 assault rifles, if you want to, armor, personal medkit, flares, all sorts of junk.
Every character has a class, and gets passive bonuses to stuff like explosives, certain weapons, melee, first aid, ect...Certain weapons can only be used by certain classes, like the medic's heal aura device.
You get scored at the end of every mission for kills and stuffs, and get presented this lovely post game graph and history of the round on a minimap. It's also a good time to draw balls and shaft and other necessary post-game rituals.
You get xp after you finish, which ups your level, improves the stats of the characters you play up to their maximums, and unlocks new equipment for you to use.
Certain missions have required items, like welders, flamethrowers, or other tools. When Techs have to hack a door, there's a little "connect the pathway" mini game that they have to play, while everyone else defends them.
There are other really kick ass features, like being able to draw on the mission map to show people where to go, being able to utilize some of your teammate's equipment (turning their sentry guns) Techs being able to hack defensive systems and use them, all sorts of stuff.
For free, it's a damn fine game.