Saw this in the theater. Full marks for both Spader and Gyllanhall for selling what is (for me) a ridiculous premise. I've never understood B&D, S&M, D&S or any of those type of games. It seems like game-playing rather than sex to me. So, the film seemed to be an exploration of what is going on in the mind of a submissive (like "The Story of "O", but better). It pushed the concept, popularized by submissives, that the submissive is actually in control.
I've never understood the concept of a body double. If everyone thinks you're nude, what is the point of not actually being nude? And if the point is to look better than you actually look, you're just setting yourself up for embarrassment with future lovers.