In my experience, Ultherapy is quite good at maintaining skin for face, neck, and upper chest, but not as good for other locations of the body.You should always consult with a board certified plastic surgeon (double check on ABPS web site) to ensure you are getting good advice and you are receiving safe surgery.
Ultherapy, when used on the thighs and knees, will provide a subtle improvement at a high treatment cost. I prefer a combination of ThermiTight and Sculptra for this area which works much better. Make sure you see an experienced physician for an in-person consultation and evaluation. Best wishes.