In general, as a bondage item they are not very effective. The problem is that thigh tapers from the top of the leg to the knee. This means that unless the thigh cuff is attached to a fixed point above the thigh, such as a belt, it will tend to slide down the leg and come off.
Thigh cuffs do have a use when someone is tied in a vertical spreadeagle position or against a St Andrews cross. In this case, they can be used to help support the weight of the body and lessen the tendency of the bound person to hang from their wrists.
A common variant available in a number of stores is the wrist to thigh cuff, where a wrist cuff is directly attached to the thigh cuff. When combined with other restraints, the use of wrist to thigh cuffs can prevent the hands (held against the thighs) from being used to fend off caresses or blows to the torso. (Wrist to thigh cuffs used by themselves could allow the bound person to stand or walk, but would be ineffective as a bondage device, as they come off far too easily.)