I bought a Cefar multi-programme one 15 years ago when they were relatively new. The TENS setting is excellent for small contractures close to the surface, e.g. sore trapezii from holding your head up on a long ride. I didn't have much success with sore quads or a gluteus medius contracture, and the other programmes such as muscle-building, which makes your muscles contract and relax convulsively, I found so disagreeable that I preferred the home trainer & so forth.
You have to be careful with the current levels or you can induce cramp, and electrodes over the blood vessels in the neck can be dangerous. Can't imagine why you'd put them there, though - to deepen your voice?