Not to be interrupting cow here, but I'll offer my view on the pillion seat I put on mine.
I bought it for my fiancee to ride with me. She's 5'6", 125lbs, for frame of reference. For short rides it was no big deal, but anything over 30 minutes and there was a little soreness. Over an hour called for some audible complaining.
I considered getting the Butty Buddy for a while, despite it's name:
http://www.buttybuddy.com/But I have decided to spend a
little lot more and get the Corbin seats with backrest.
http://www.corbin.com/re/I've seen people complaining that the Corbin seats do not quite fit the classic look, but have not heard anything about comfort. I will probably report later this summer on the comfort aspects of it.
Hope this helps!