Most of the popularity I would say is from the hype. Everyone is talking about so more and more people start to read it.
I'm reading them now, and find them pretty entertaining. It's not the most beautifully written book of course. But I actually love the story so far and I love that someone wrote about something we would usually consider taboo.
Christian (the lead male role, aka Mr. Grey) I find to be an intense, deep character, which is what keeps me so hooked. I want to know more about this character and why he is the way he is.
To me the book is so much more than sex. That is just normally the main focus for those that are talking about it.