Good review!
I'm about halfway through world three at the moment, don't know if I'd say it's a 9/10 game for me (although I'm also pretty hard to please in general) but it is indeed gorgeous and full of personality. The craft aesthetic is pulled off better here than in any other game I've seen.
I actually find the co-op in this harder than in NSMBU, though; there are quite a few spots, especially in hidden levels, where there's not a lot of solid ground to stand on and my partner and I ended up just taking turns attempting the levels.
For me it's probably about a 7.5/10 so far, maybe an 8, which I'd still consider very good.