I find it extremely hot. I've made it a habit to have any girl whose feet I've tickled tickle mine back.
For me, this is erotic for a few reasons:
1. Women touching male feet seems more taboo than women touching penises in Western culture. It's sexy because it's counter-cultural.
2. Even a woman who hates feet has a high chance of tickling them if it means getting back at the person who previously tickled them. It's sexy because it's someone going out of their usual comfort zone and actually enjoying it.
3. It completes the tickling relationship: I've tickled her and she tickles me. The result is a situation where neither person feels emasculated/objectified or humiliated. It's sexy because it's equal.
To date, I've had this type of tickling experience with close to 40 women over 4 years of college. I think it works mostly because of point #3, generally because the ones who aren't very ticklish don't seem to enjoy either the m/f or f/m parts as the ones who are more sensitive.
Being a switch, I love seeing big, strong, dominant men, reduced to giggling piles of goo. Just as much as I love being dominated by one. It all depends on my mood and the "moment" the oppertunity appears.