I run 91. Always have. I'm with Matt on this one. The few cents extra per fill up is worth it.
I ran 87 once just to see if there was a difference. And it could have been my imagination but it seemed like it was harder to start the bike once it was already warmed up and it seemed to take longer to warm up than with the 91 octane. Only advantage was I did get about 4mpg more with the 87. But until I get to a point where spending an extra $.90 a week becomes a problem, my baby only gets the best!