I have Spanx and LOVE them. I am "skinny" BUT after two kids, there are definitely areas that need some tightening. I wear mine whenever I wear clingy or fitted dresses and skirts. Makes an AMAZING difference in my figure! As for them being too tight..... well, sure, if you get too small of a size. You wouldn't wear a size A pantyhose if you're weight/height says you should wear a size C, would you?? All you really want to do with the Spanx is smooth out the lumps and rolls, not cut off your circulation.
I got mine at Nordstrom's (only because I was there with a friend and she said I had to try them) but I've seen much better prices on Ebay. Not sure what other Dept. stores carry them. I'm not sure about losing "two dress sizes" but they surely do help fix the imperfections so that you don't look "lumpy"! I recommend them to all my girlfriends and recently told my boss about them for her son's wedding and she's hooked now!