I actually read the Fifty Shades of Gray (Grey? whatever) books. All three.
Biggest. Waste. Of time. Ever.
First of all, the writing itself is pretty awful. If you get past that, the characters are just...fluff. Nothing really unique or meaningful about them. The story line would actually be sort of interesting if all the sex scenes were taken out (which is why I actually read all three, to find out if they ever got married).
That's just the writing itself. As for the psychological side...
I think a huge part of the appeal of these books is that, as persnickety said, the guy is "mysterious", a little dark and dangerous...but as the story progresses, the girl is able to change him, to an extent. To something she can handle. So basically, she gets to play with fire, and tame it.
It's all pretty messed up, though had it been written better, and without the pornographic junk, it could have been, if nothing else, mildly interesting.
But that would have defeated the purpose, because I'm pretty sure the point of the books was the sex, the fetish...to sell copies.
At any rate, I absolutely agree that it's garbage and I find it sickening how popular it got, and that there's a movie being made.