They've had it at this price before. Fantastic game, tons of content including 100+ levels and supports 4 player local mp, online leaderboards, a genuinely fun touch screen mode for use in portable mode, mini games such as soccer and several playable characters as well as my personal favourite; rythm stages which are simple and generally fairly short, but absolutely fantastic nonetheless.
Totally worth it for $29 if you haven't played it before. I've only played the demo and a big on XBONE so I'll be picking it up today.
Ooh will grab this for when I finish Mario!