Arguably a perfect game, no other co-op game blends action and cooperation so well, and the AI director means that every run of a campaign is going to be different in some way. Versus is great as well, and while you may run into parts of some campaigns that are skewed towards one team, it's incredibly balanced in the overall picture. The level design is amazing at showing you where you need to go without being overbearing in guiding you, using landmarks and lighting to steer players. If you have enough people for co-op or Versus mode you have GOT to play it, it runs for like 2$ on sale all the time.