Sensa is definitely a scam. Even if some people have lost some weight with it, I know that LOTS of people have lost a lot of MONEY with the scam they run. They tell you they will give you the first month free. But they send you TWO months worth, and the only way to not get charged, is to turn around and send back that second month immediately.
However, people are getting charged on their credit card even when they send it back.
Don't do it.
I generally figure that if it sounds too good to be true, it probably is. I don't know of any weight loss aids that work without some effort on the part of the user. I think I'd save my $ and work on starting a program that implements a healthy diet and moderate exercise.