Random thoughts:
Single Player Content has felt really lacking. 'Classic' Mode blows. The Crazy Hand / Master Hand feel like they're ripped from mobile / MMO design for 'daily quests'. The challenges are AWESOME, but I ripped through them in like an hour. No more? Come on! The home run derby / angry-bird target stuff is boring (I don't mind they're there, but they aren't fun.
The core gameplay is fun and I enjoy the new characters, particulalry mega-man, pac-man, and Little Mac. But the environments sort of ruin it. There is WAY too much going on in almost every stage and there is very little tell when a stage transformation is about to occur. I almost want to autoquit when certain stages are selected.
I haven't tried 8 player cause I don't have the adapter yet, even though I have the controllers. I'm considering picking it up or trading in the game for Captain Toad.