It's fun and a pretty good incentive to move about. That said, we were pretty bored with the included songs and to have access to all the songs you have to get a monthly subscription which was a bit too much for something that's just fun.
I used to like the series from 360 to wii u . It was a nice way to get cardio in. Lately though, money grab, meh choreography and songs i dont vibe with killed it for me.
I think older just dance games, dance central spotlight are better.
My wife is a big fan, drawn in by the songs she recognizes and the promise of exercise. But ever since we visited Japan and tried the rhythm games in the arcades there we've both felt Just Dance to be a bit lacking -- the tracking isn't very sensitive and has a huge tolerance for accepting wrong/mistimed moves
I feel like it's one of those things that's super cheap to make. It's basically just repackaging the same core mechanic with different song and visual packs.