Fantastic co-op. There are some mistakes that are easily made - especially around the deck gun (no diagonals!), hot spots (there's a limited number for each game), and similar - but well worth the time and money. Makes a nice change from Pandemic; very different style of game, and the "alpha geek" problem (one guy telling everybody else how to solve the game) is much smaller in this one.
Solid, and the de-facto classic cooperative board game. Well worth putting into your collection if you're tired of "everybody for themselves" types of games (or just want to play something that breaks out of that mould.) The base game on its own is well worth the money. The "On the Brink" expansion, though, adds so much to the game, I'd rate it a must have if you get the base game.