My fiancee/stoker is very happy with the Terry Butterfly, but BE WARNED--it's not all about the seat. She started out on the Butterfly and couldn't get comfortable at all. We went through 2 other saddles before I played a bit with the saddle height, fore/aft adjustment, etc and bar reach, and now we're back to the Butterfly knocking out pain-free 40-50+ mile rides.
I also put a Rock Shox suspension post in the stoker seat for the times I don't say "bump" fast enough... Just fine yourself the $100 for being too slow and your stoker will thank you
DrPete