I haven't worn Fitflops, but I've worn a similar brand that claimed to help you get fit. I didn't notice any results, so I quit wearing them. A shame considering the price you pay for them.
They've done market research showing that the fitflops and shapeups don't do a darn thing to help your physique (just google, you'll find a lot), but you may still find them comfortable.
I've been wearing fitflops for quite a few summers now. I have several pairs and just love them. They don't really do anything for physique i don't think but i find them extremely comfortable. I have quite sensitive feet so if i don't have the right shoes, they get painful relatively quickly. But with fitflops i can walk for hours with no problems.