My 11 year old plays DDR daily, and he is now skinny (not too skinny, just right), whereas before he was just a little tiny bit round. He has been obsessed by DDR, in that he talks about it, plays it for hours (if allowed) until the sweat is pooring off of him, and he memorizes every song, every artist who does each song, etc. He is extremely good at it.
DDR has been an obsession, but he's great now about playing DDR for an hour, then jumping to other activities. We also have taught him that it's great to talk about DDR, but only with someone else who is equally interested. Unfortunately, there aren't that many who are as interested as he is!
DDR, if used correctly, can be a great social tool, if other friends are interested in it as well. The only trouble is, when your child plays it almost continuously, the other friend may get frustrated because they just can't compete. I understand this too -- I am overweight, and I play DDR with my son so that he's actually playing "with" someone, and so that I can get some INCREDIBLY good exercise. I'm lousy at it, but if I did it daily like my son does, I'm sure that I would improve. I think it's a great game!
Kris