Yeah, it's fantastic, there's really no other game quite like it. It throws you in this ultra-difficult complex world and aside from a simple tutorial at the start (which ends with you dying, just to get you used to the constant failure you will face ahead) the game doesn't explain anything to you and just pummels you relentlessly until you eventually power through it or quit in defeat. You never feel alone in your punishment though and it's all of the online stuff that really makes the game a classic. Reading the notes other players leave behind - some of them helpful, some of them deliberately trying to lead you into a trap - is addictive, and being able to watch the ghosts of other players meeting their ends is one of the coolest online features I've ever seen in a game. There's nothing quite like the feeling of entering a room for a first time and see it littered with the ghosts of people who died there before you, immediately letting you know that you too are about to meet a very grisly death.
If anyone wants to see a really compelling argument for why the game is so great, definitely watch this video from Gamespot where they explain why they chose it as 2009's game of the year:
http://www.youtube.com/watch?v=YgYBivaVco8
