I did not like the Spanx at all. I even thought they made me look worse, not better.
I attributed that to...
1) I'm only 5'0" - Spanx comes in only a handful of sizes so the XS that fits Tanya who's 8" taller than me is going to fit her a lot differently;
and
2) I'm not overweight at all... but I have the "mother's apron" - loose skin from childbirth 14 and 16 years ago. According to my doctor, no amount of weight loss will improve this; only surgery will. Which, I'm not about to have! I will only go so far for the sake of fashion and beauty... sorry!
I have tried numerous brands of shapewear (many still stashed away in storage somewhere - I probably should get rid of them) and haven't been happy with any of them. I also don't like that feeling of constriction. I get more self-concious, not less (which should be the point). I don't mind control-top pantyhose and a cami though - a little spandex is okay. Too much is too much, and tends simply to move lumps from one area ... to another.
Meanwhile, I've learned instead to choose carefully to camoflauge.
The sweater dress? It's not really that clingy... but, I have found that it looks a LOT better if I wear a simple VS negligee - basically a full slip - underneath. This acts to disguise lumps and bumps and give a smoother line. I have a very pretty one too... and I can even sleep in it so it does double duty.