Mario Sports Mix can be fun, however it has a few disadvantages. Luckily, basketball, hockey and online mode make it up.
Advantages:
1) Great graphics: everything looks brighter, prettier and clearer compared to previous Mario spin-offs (e.g. Mario Kart, Mario Strikers Charged)
2) Basketball and Hockey: these two games can be really competitive, especially if you like playing against friends or online
3) Dodgeball is surprisingly good.
4) Online mode: this is the best feature of the game with no doubt. You might have unlocked and beaten everything in the game, but you will always come back for more points if you get easily hyped with online gaming :D
Disadvantages:
1) Volleyball: this is definitely the worst Mario sport I have ever played. It is slow, boring and it takes forever to complete the tournament modes.
2) Roster: I cannot blame Square-Enix for including some Final Fantasy and Dragon Quest characters in the game, since they are the ones that developed it, but how dare you exclude such classic characters as Koopa Troopa, Hammer Bro, Dry Bones, Birdo, Toadette and even some more recent but great ones like Rosalina, Funky Kong, Dixie Kong and Lubba?
3) Artificial intelligence: the tournament mode is going to end up being really boring especially in the normal difficulty.
4) Lack of alternative camera angles: it is always nice to give the player the opportunity to choose the camera angle that fits them the most. A camera angle like the one in Mario Strikers Charged would have been awesome in this game.
5) Inability to control well all of the players in your team: even though you can choose the automatic setting for when you change your characters, sometimes (e.g. in Volleyball) you are limited to being a simple player in your team.
Overall, this game is worth it if you are a hardcore Mario fan. Nevertheless, Square-Enix could have done a lot better with this one.