I used one after both pregnancies. I always got complimented on how flat my stomach looked so quickly and fit into my pre-baby pants by the time I went back to work at 8 weeks (after gaining 50# each time and obviously they were a little snug).
You do "crackle" just a bit from the big velcro strip and you'll have to adjust it when you stand up from sitting. I prefer for others to not know about it so that was always a little awkward trying to cover it (but I'm super private sometimes)!
They're expensive, but I figured it was the price of 1 or 2 pairs of pants or jeans and if I could wear my old ones, it'd pay for itself!