The Wellmont Wellness Center in Bristol, TN has the RealRyders. They are alot of fun to spin on but I can't imagine anyone falling off one. They have a wide base of support and I have seen some big guys leaning side to side on them. No one has had a bit of trouble that I know of. They have clip-in pedals or cages (depending which you choose).

That being said, I am not particularly crazy about these bikes and for my DH, the side to side motion of "leaning" hurts him you know where. The seats are not particularly comfortable for me and I have never felt really comfortable on these spin bikes (nor any other spin bike, for that matter) They surely do feel as though they are giving a core workout, but only when you are leaning. And you are not leaning during all of the hour in the class. I think that fitting the Realryder is hard and I never seem to be comfortable despite trying to adjust it every time I go. The resistance is very easy to set though I found a lot of variation from bike to bike. One might take a complete turn on the knob to increase the resistance and one might take a quarter turn.

All in all, they definitely add to your core but are no substitute for a real core workout.