It took me a while, but I toughened the fuck up and beat the game just a couple weeks ago (FYI I bought the game on release day back in September). Like many others, I've only been able to play the game in ~15min increments because of the horror.
This is what I think makes the game so successful. When you have a game that's so damn good at getting under your skin psychologically, and yet you keep coming back to it because you want to know what happens next... it's brilliant.
Would people who finish this game want to play it again? That is, what's the replayability? Even if you know the story, would you play again?
Amnesia doesn't really have any real replay value to speak of. All there is to this game is a single campaign - that's it. No unlockables, nothing. You could play through the game again to see the different alternate endings, but they're nothing worth redoing the whole game for.
I'm still a huge fan though, and maybe after a little while I'll go back to it and play it through a second time, just for kicks.