Well, as a reader I'm strictly a voyeur. I'm watching the action rather than being a participant in it. It's a very rare thing for me to actually feel like I'm IN the action, as it were.

As a writer, on the other hand, I try very hard to actually be in the POV character. I feel like I write better when I'm strongly within that character's head, especially for writing sex which is always a challenge. I feel like my smut gets too clinical when I write as an observer.

Of course, whether a reader or a writer I'm more focused on the emotional connection between the characters than on the actual mechanics. I generally don't need minute detail to figure out what they're doing physically. A vivid imagination is a big help here! *laughs*

That said, a lot of the time in slash stories I skim over the sex rather than poring over it intensely. As I said before, I'm there for the characters and the development, not the porn. Unless the porn has something in it that shows how the sex is changing their perceptions / lives / relationships (or confirming it) it's just not that interesting to me.

Hope that helps or at least adds another data point!