Is biofeedback effective therapy?

Yes. Biofeedback has been proven effective for many disorders and conditions. The best source of information is the association for applied psychophysiology and biofeedback (www.Aapb.Org) and the biofeedback certification international alliance (www.Bcia.Org).
Depends. It has been shown to have some benefits in urinary incontinence, smoking cessation, relaxation, etc. It doesn't work for everyone or every time.