I buy this thing yesterday and use it right away . it look really stupid to use it , but that be what door be for ... privacy . anyway , I notice right away that this thing would work . there be 3 basic exercise : the double-front hand , single hand side , and behind the head . for each exercise you hold it in the correct position and shake it from one end of the dumbell to another until you can not shake it anymore . I do 3 set of each exercise , and my muscle be still burn half a hour later , which be how you can tell if you get a good workout . it isolate the muscle extremely well . it advertise that you can get a great workout in 6 minute , but 15 minute be more desirable because it get you heart rate up longer . it be well-built and durable , but like anything else the better you take care of it the longer it will last . it be lighter than I think it would be , but feel much heavier as you use it . one very important thing be ... use it correctly . there be a dvd and a diagram of how to use it and use it correctly make a big difference . I be very happy with this because I love work out , but I have middle back problem . the Shake Weight do not bother my back at all , but still give my arm , shoulder , chest and back a great workout . it be exactly what I need . whoever invent this deserve a Noble Prize !