Well, I went commando-Spanx. It was so comfortable! And my dress looked sooooo slinky and lovely. Unfortunately when I sat down the waistband of my tights promptly rolled down my sleek Spanxy middle and proceeded to strangle my waist. Owww. Totally ruined the line of my dress, as I discovered when I escaped to the loo to rearrange my tights.
Still, I am converted. (With long dresses. With short, not so sure - for fear of the Spanx-Flap)