Classic Steam game with a mountain of achievements and unlockables. If that is something that drives you... you will be busy for a bit.
Team work is a MUST. The pain of having someone run off becomes very clear very quickly. Just like in Left4Dead. The character classes work well, although some levels require certain classes be present. This is a game breaking mechanic. If one player dies... that happens to be the required class... the level is over. Regardless of the health of the other players. Yes, I can hear you saying that this means you must protect that player. Escort missions are only fun for so long and there are some balance issues.
The aforementioned balance issues revolve around some of the enemies. Okay, one enemy; the "parasites". These little buggers kill you way too quickly. Extensive healing is the ONLY way to rid yourself of them. Maybe add a new melee option to smack them off of your squad mates.
I would have been happy paying $10 for this title. The fact that it is free is even more amazing. Please Valve... use this as an example of how quick people will pounce on Co-Op gaming. We need more of it.
Game Play video: