One of the best co-operative boardgames out there, provided you don't mind being eaten alive. Arkham Horror takes the horror stories of H.P. Lovecraft, August Derleth, Clark Ashton Smith, Robert W. Chambers and Brian Lumley, mixes them all together and creates a sort of evil RPG version of Monopoly, except that instead of buying property you're closing dimensional portals. And you can move in any direction you want, but there'll probably be something chasing you.
AH is a sort of 'random-RPG-quest-in-a-box', in much the same way as Talisman but which much more variety and content. The game has quite a few RPG trappings, such as character sheets, hit points and so on, which mean it might not be one for the kids. It is, however, definitely one for players who'll get a buzz out of the atmosphere of Lovecraftian futility that the game quite often creates. The Arkham Horror experience is one of gradually losing control. At the start of a game, it usually seems like it'll be easy for the players to keep things under control, then as the turns roll on the board descends into chaos and it becomes a question of whether the players can grab what they need to survive when the nightmarish Great Old One rises from Its millennial slumber. Actually winning a game of AH can be a bit of an anticlimax, and the best games I've played have ended with me and my comrades snatching defeat from the jaws of victory at the very last moment.
The game features huge numbers of counters, markers and decks of cards, which can get fiddly at times and might intimidate n00bs. But actually it's not nearly as complicated as it looks, and you could probably afford to lose at least half of it before the game would become unplayable. Once you remember the order in which to resolve everything, all the game really consists of is running around the board, drawing the right-coloured cards, and following the instructions on them before MEETING YOUR INEVITABLE DOOM. Seriously, I can't overstate how much doom there is in this game. And if you get bored of the amount of doom, you can get the expansion boxes, which add loads more doom and about thirty more decks of cards.
In summary: almost freakishly awesome. You may be intimidated by Arkham's apparently complexity but you will come to love just how much stuff there is to do. It's tremendous fun once you understand it, and there is a certain sprawling beauty in its arcane fiddliness.
P.S. This game comes with six dice in the box. This is bonkers. The optimum number of dice needed is probably somewhere in the region of five bajillion. Just a heads up.