I have heard the same thing about them as the other posters - don't rely on them. They may work in theory, but I would not trust anything that is not bolted to the wall. I doubt there are any suction cups that can really hold the weight of a person, especially on a wall. A too smooth wall will have trouble with slipping and a too rough wall will not give a good seal.
Many of the WDW resorts have put grab bars in the tubs for all first floor rooms and it is possible to get an accessible room with a roll in shower or one with a regular shower/tub and grab bars.
As Bill mentioned, they are not always where you want them to be (they will meet ADA guidelines which are very general to try to meet many conditions, but may not be best for an individual).