The first iteration of an amazing concept that was bettered in every way by its sequel. In retrospect, it was honestly a bit scummy for Valve to ask for $20 each (as I recall, might have been more) from you and your friends when L4D2 was just a year away, but it's still a great game regardless. However, there is literally no point in playing it these days (unless you're a speedrunner or something, I guess), since L4D2 has all of L4D1's content.