There is no great research on this.  In my opinion, they don't help  because it isn't the muscles that are sagging!   Sometimes, they even make things worse because some wrinkles are caused by excessive muscle activity - think frown lines and forehead lines, for example.

 Sagging skin is caused by many factors but the main ones are loss of collagen and elastic fibers in the skin due to too much sun (mostly) and aging (a little), loss of volume from shrinking fat pads and bone which support the facial architecture, and sometimes from weight changes, expecially if over 40.  Dr. I