Steam recently held a free weekend event for Payday: The Heist, an online co-op shooter about robbing banks and other such missions in the life of an organized gang of criminals. It plays a lot like Left 4 Dead, except instead of shooting zombies, you're shooting cops. Instead of getting incapacitated by hunters and smokers, you're getting incapacitated by special forces units called the "taser" and the "cloaker." Instead of having to make it to a safe house, you're (usually) trying to break into a sealed vault somewhere.
Payday has enough distinctions from the L4D series to make it feel fresh and interesting, and could certainly provide enough hours of entertainment to justify a purchase. The RPG-style leveling mechanics alone could ensure that there's enough incentive to stick around, but after four hours of playing, I wasn't quite ready to buy it. It's a really good game, nevertheless, that you still might consider checking out. More of my thoughts after the jump.