I have bought one! Went to Debenhams today in a mad panic because I have my dress fitting on Tuesday, need THE underwear for the day and have felt abysmally ill all week so haven't bought anything. Anyway, I lined up 3 in various sizes around what Bravissimo measured me at last time I was there, and found one that fits! The effect is *COFF* interesting. It seemed to make my boobs a tiny bit smaller (fine by me!) but look fantasmagorically pert. I even did my own little bounce and wiggle test; true to word, the thing didn't slip down or require any 'hoiking' at all. I am rather impressed. It's not at all comfy to wear, but that's a small price to pay for how perfect it is!
In case anyone's been eyeing them up, you can actually get them in a lacy version as well as the plain black and nude colours. Actually looks kinda pretty in a scaffolding sort of way!