I have never used one, but I am very skeptical. I know in my younger days, we had similar items available to wear (corsets) and they gave me the worst pains and cramps I could imagine, plus it is very uncomfortable just moving around in an average daily fashion, like getting up off the couch, or if you need to sit and the only place if a futon style seating. Well, just stand because trying to sit down, stay seated and then get back up will be difficult and painful.
Plus, do realize, it does not "train" your waist, it simply holds everything in place while you are wearing it but once you remove it, everything will return back to where it was, the higher fat content you have, the faster it will fall back into place, so to speak.... If articles of clothing really worked at getting something in place, boobs would never sag.