Does anyone know biofeedback devices work?

Biofeedback operates on the notion that we have the innate ability and potential to influence the automatic functions of our bodies through the exertion of will and mind. Biofeedback has recently been shown to give us what had previously seemed an impossible degree of control over a variety of physiologic events. It is a great tool to harness the power of mind-body medicine. Source(s): http://www.holisticonline.com/biofeedbac...

Call the church of Scientology. They use them all the time.
They work if you are perceptive and willing enough to pay attention to your body and take the action to retrain it. You can control your sympathetic nervous system responses to stress.
Sports trainers use devices like these all the time; I doubt they'd spend the money or the down-time on something they thought was a scam.

The Resperate system is getting good press. It's used for stress control & high blood pressure.
http://www.resperate.com/