Can't help but think this one's a step down from first game in every way possible.
Story is neither about Mario nor Rabbids - it's about some black goo threatening galaxy. Rabbids now talk, but that makes them lose the only somewhat unique trait they had, meaning you can replace them with anything else and lose nothing. Gameplay is largely the same, but all stats and special effects were moved from weapons to skills and Sparks, so all weapons are now purely cosmetic. But the worst offender is the interface: navigating multiple menus just to switch Sparks or characters gets annoying really, really fast. Or using the "X" key to sprint - what were they thinking with this?
This game is still perfectly playable, but nearly nothing about it is enjoyable, especially compared to the first one.