I haven't been to one so I can't say if it really works or not, but it sounds too good to be true. Usually weight loss from spas/saunas/etc is from water weight.
From Livestrong:
Reputable health experts — both traditional scientists and alternative medicine specialists — remind consumers that a body wrap is not a weight-loss quick-fix. Because all the weight you lose from a body wrap is the result of dehydration rather than fat loss, the pounds and inches will return within a few days, once you have re-hydrated. The U.S. Food and Drug administration reminds consumers that “fat doesn’t melt,” and any product claiming to “melt” fat or cellulite is making unsubstantiated claims. The Healthy Weight Network advises that you can spot a fraud if they claim quick weight loss without dieting, use testimonials rather than scientific studies and make claims that aren’t listed on the packaging.
