These flip-flops are the most comfortable flip-flops I own.  I wear them everyday as house shoes and to go out on the town.  The only downside is I got them in white so they're looking pretty dirty from all the wear.  I haven't noticed a huge difference in tone, but I'm pretty fit as it is.  I did notice that when wearing them to do my lunges around the house, that my calves burned like never before though.  It forces you to use more muscles to balance.  I got another pair on my wishlist and will not go back to flat flip-flops.