You can feel it doing what it claims to do but whether it has long term effects is unclear at this stage. I'm tall so bought the largest size but I find it almost impossible to put on without assistance and it is more detectable under clothes than the maker claims but having said all that I'd be happy to recommend this product with these few resevations.
It was too large - by the time I had adjusted it, I had run out of velcro and it was cutting into me under the arm. If I loosened it off at all I found that I could sit how I wanted and it wasn't correcting my posture effectively.