I'm in the "Have not played, will not play" category.
I think it appeals to the same people who liked the gameplay mechanics of old-school adventure games and CRPGs (think Baldur's Gate for the first 3-4 levels of character, and older stuff) -- the people who want a game that is so hard that they have to work their asses off and get really good in order to beat the game. It's less about story/character/whatever and more about being really, really good at the gameplay. If you stretch this out, you get people who actually play I Wanna Be The Guy and win one day.
I don't like hard platformers just because I don't really like having to trial-and-error grind levels. I think winning by a lucky shot isn't winning at all, and I tend to ragequit if I can't finish any game's level after...20 or so tries. I can try for a while, but spending hours on a level and getting nowhere starts fraying too many nerves, and when nerves fray, I do worse at a game, and then...it just sucks.