They will work on the first generation Prius, '99 - '03, but not on the 2nd generation, i.e. '04 to present. The first generation had a steel body, which will support the weight of the rack plus bikes, whereas the 2nd generation body is a combination of plastic (fiberglass) and aluminum.
With one bike, and a light bike at that, you'd probably be okay. Otherwise you risk damage to the 2nd gen. Prius body work.
Rick / OCRR