Anyone know how good units such as this are?

http://www.ebay.com/itm/Vehicle-4-Parking-Sensors-LED-Display-Car-Reverse-Backup-Radar-Kit-System-/300914966845?pt=US_Relays_Sensors&hash=item460fee013d
Or if there are reasonably priced, easier to install - but just as effective - sensors?

How difficult is the install? Anything easier in terms of installation? I've seen the license plate sensors, but not sure of effectiveness.

any help is appreciated