Arkham Horror is one of the best games I have ever played. I've enjoyed this game for years, and at our store we've played in last summer's Arkham League, as well as field ing a team in the current league.
Arkham Horror is, essentially, a Cthulhu Role Playing game, but without having to take the time to come up with a scenario. All of the players are working together to defeat the game itself. It can be played with 1-8 players -- yes, the game can be played solo -- and you can actually have more "investigators" than players. When playing with more than 4 players, the game difficulty increases, but that does not dimish the enjoyment of the game.
The rules are pretty complex, but only until after you've played a few games. Most of the rules start to come naturally the more you play, and after the first few games it'll only be specific issues that will cause you some problems. The game can take between 1-3 hours (sometimes longer), and once you are familiar with how the base game works it actually becomes a little easy to defeat the Ancient One. The expansions actually add to the game, and makes for more variety in game play experience.
I highly recommend this game to anyone who is remotely interested in it. You can always check out the Arkham Horror forums (where I am also a poster) to learn more about the game, and also learn that the Ah community is also a very friendly one willing to help out new players.