To me, it's a category all it's own.
We've just added a new Tranny site to the
www.ARS4Real.com line up too called Rate My Tranny -
http://4real.ratemytranny.com/frs.php?c=free
The site looks great and is ready for promotion. Personally, I'm amazed at how good looking some of these tranny's are. The tranny in the second free video trailer on the front of the site has a beautiful face... it kinda surprised me that she's got a cock too. ;-)
This niche has always converted well for me...I've promoted SheMale sites via SE's and I'm not sure if it was more so gay or straight traffic that clicked through - maybe it's both, if for no other reason than a surfers curiosity.
I'm not sure why this niche does so well, but check out RateMyTranny and let me know what you think.