I can't say to how these work, but I can talk about the fit. I'm 5'7" and 140 lbs, and I'm thickest around my bottom and thighs. I bought a medium in this and should have bought a large, the holes for your legs were awfully tight! I normally have trouble with shorts being too tight around my hips, not with the leg holes being so small. I hope this helps others find their fit!
They did feel very smooth and would be hard to detect under clothing, I'll have to buy the large size one of these days.