Best Mario since 64.
Moveset is fantastic, music is good.
The collectibles, moons, are a major sticking point. There's over 800 of them (technically a max of 1200, I believe, but 830's the "official" max). A lot of them are gotten by braindead means, such as "wear outfit and go to place", "ground pound in glowing spot", "buy one at the store" and so on. There are some that are quite challenging but far too many are braindead. A big part of the game is literal filler, in other words.
Another big problem is the transformations. Mario's moveset is fucking amazing, and the transformations instead restrict it to "two variations of one shit move". It's a lot more fun to sequence break around required transformations than it is to use them. It's the same problem that Sunshine and Galaxy had, perhaps not as bad as it's optional, but still a problem.
I maintain that you should never pay more than $20 for a game, but it depends on you. How much is the sale, again? I'd say $30 might be worth it if you like platformers, otherwise I'm not certain.