I'm going to have to agree with Mariposssa.I don't see using a game system any different then using a dvd or inexpensive home gym items,ie; bands,weights,ball,etc.As long as you use it to the fullest potential and on a regular basis...you will see results.That being said,many people don't use it that way,because for some,it's not as encouraging as going somewhere and working out with others,or they don't view it as a workout and wont use it to the fullest potential.
I finally did buy the Zumba Rush for Kinect,and let me tell you...I am out of breath and sweaty when I am done with a workout.When I use it consistantly,I do see a benefit in my clothes and how I feel...the key word there is consistantly though!