Really fun game, just like the first Super Mario GALAXYYYYYYY!!
They kept all the good bits and added some more, like the improved second player features (not just collecting star bits from the sidelines.. now you can actually kill some baddies dead), and of course more of the brilliant level-design we saw in 1. Bottom line: fun.
Such a fun game. It really redefines platformer games. Also fun to play with a friend with the improved 2-player functionality. However, it is a quick game to beat, but also has side missions that extend gameplay.