I have been wearing fitflops for 2 weeks now and there is a definite difference in the muscle tone on my thighs and bum,in fact my bum has shrunk slightly.
I have had them on pretty much constantly since I bought them so have no idea whether they would work as quickly if you only wear them occasionally.
I have had my family in fits of laughter by making them put the fitflops on and then stand on one leg,it really is difficult to balance like that in them until you get used to them. :-)