I first discovered these a few years ago and I haven't worn a dress or skirt without them since.  My thighs aren't overly fat but they're not thin in any sense of the word and I have found that these prevent chafing and feel light even in the dead of summer.  They can create a tummy roll when I sit down, but they are durable and have withstood several washings.  If the leg of the panties is too long for the dress/skirt, I simply fold the legs up to the right length.  Overall this is my favorite Spanx product and an essential in my wardrobe.