You know my experience with the game has been about as equally depressing haha. It's really not much of a game. However, working at Gamestop every person that I've talked about it that has bought it and played it seems to love it. Obviously almost 90% of them are females, but the absolutely love the game. Actually the game is pushing sales of other dance games such as DDR because they want more like it (even Karaoke as well). I always have to ask why they like it, and the majority answer is they get to listen and dance to music they like with friends and family around.
Now as I said, I didn't think must of the game and really thought it was quite poor, but I think we can all relate to that sentiment. I'll say it pretty truthfully that I think the Guitar Hero and Rock Band setup is dull and overused and really a silly way to express musical notes and rhythms. Even moreso, I'm really not good at playing it (smaller hands and carpals tunnel slows me down). But why do I like playing them, and will be getting Green Day Rock Band. Well I get to listen and sing to music I like, and at least for once in my life I get to feel like I'm somewhat of a rockstar. And even though I'm not amazing at it, it's not about getting high scores for me.
I think this is how people feel about Just Dance and its something we can all relate to. It's not about playing a perfect game or being the best, but just having fun with something you are enjoying and having fun with. And if I think people look at it that way, they'll think a little higher of the people enjoying it. Don't think of the developers too highly, because they could have done a much better job, but at least understand why they are enjoying it just like for my reasons of enjoying the GH and RB games despite being silly when you think about it.