Goofy, silly fun. The dialog is funny, especially if you are somewhat familiar with games and the previous Banjo games in particular, and most especially if you love nut jokes. The plot isn't important, and in fact the platforming isn't really important. Sure, you run around collecting shit, which I do appreciate, but the meat of the game is building vehicles. My son complained that I spend more time building vehicles than playing the game, which is very true. I could point out that he spends most of his time flying around in a crap airplane I built than play, too, but I don't.
I'm not going to bother even mentioning the plot because it is stupid. Not in an annoying way, just that it's something I don't care about. The important bits are that you enter various worlds and complete various challenges within, earning various McGuffins to open up other worlds.