I've been running my SS for about three weeks now, and am new to the game. I find it provides a balance to my riding. I do a two-hour ride on the SS and feel I'm benefiting from the workout I'm getting, then I jump on the FS and feel more confident than ever on the downhills, as it feels that bit more capable than the bike I last rode.
I'm loving the fact that with my flat/SPD pedals on the SS, I can bomb down to the shops on it, nip into town or go off on a two-hour ride (which is the fitness-equivalent of a three-hour FS ride), when I haven't got the time for a longer ride.
Do both SS and FS – thoroughly recommended.