I have have my Shake Weight for man for 2 day now and so far I really like it . the 6 minute dvd workout really do make you muscle burn ! and my arm definitely swell up after ward just as if I would be lift weight . I can not say what the effectiveness will be in the long run but I do not see how this could not beef someone up . one thing I 'll say be that I do not really feel a workout in my chest . I may have feel a little soreness there today but not much . the workout dvd go over 3 workout and the Shake Weight come with a card show those workout plus one more that be not demonstrate in the video . my plan be to do all 4 every day . maybe I will notice it in my chest more ? I do have concern that this could hurt someone back if they have have previous back injury but it do not seem to be bother mine . overall it be a well make product and I think it be go to do a good job make my arm a little more cut . and honestly it be a lot more fun to use than plain old lifting weight . 4 Stars ! oh and I notice someone else 's review say just to buy the female shake weight ... I do not get that because the one for man be 5lbs and the female version be 2.5 lb ... 11\/14\/10 Update I still think this be a good product . if anything I keep slip up and stop use it for a week or two . but my arm be definitely more define . especially my triceps ! I have never be able to see any definition in that area but they be bulge out now when I flex . I be go to try to stay devoted to do the workout twice a day now and really accelerate the muscle growth . a quick bit of info . . at first it may seem that it be slim down you arm . what I have notice be that it burn any fat off first and then the muscle growth begin . do not be discourage . even use it off and on I be definitely more tone than I ever be . I be gonna push this thing to the max and see just how rip it can make I .