It's a lot better than NSMBW. The level design is more spacious, satisfying and above all, accommodating for the multiplayer aspect. The Flying Squirrel Suit is the best jumping/flying powerup in any Mario sidescroller, because it manages to be satisfying like the Cape or the Tanooki Suit, but without giving you the ability to skip nine-tenths of the game. The boss fights are a bit more challenging (and harder to cheese), and the music is a bit better than NSMBW (though not by much, unfortunately). Oh, and the team in charge of the art clearly heard all the complaints about the New Super Mario Bros. series having drab art, because they kick it up a few notches in this game.
All in all, I'd say this entry is the best in the NSMB series, and definitely one of the best Mario sidescrollers.