Mario Party 9 does right what Mario Party 8 did wrong.
Mario Party 9 gets the balance right- motion control is never vigorous, but used in interesting ways in minigames. Also, figuring 10 mario parties were a bit too many, Nintendo drastically changed the rules. Part of Mario Party 8's problem was that it was too similar to the other mario party games.
In Party 9, all players travel in a vehicle alltogether along a one-way board. When someone gets a six, the vehicle- and all of the players- will move six spaces. Byt the space landed on will only affect the person who threw the dice. The aim is different too: instead of collecting stars using coins, mini stars dotted around the board in clumps of 2-10 are the prizes, and they are also the prizes for minigames- no coins in Mario Party 9. And the player who rolled the dice is the person who collects those mini stars when the vehiclepasses through them. Once the vehicle reaches the end of the board, a boss is fought, and the person who dealt the most hits wins mini satrs. This change in formula makes Mario Party 9 feel radically different from the rest of the mario partys. It's also what changed my scores for this game from a 5 to a 7. Each party board has a different theme and several twists that make it different and enjoyable. There is chance in Party 9, but it rarely angers because there's more strategy on the boards with the new formula, and the minigames are great skilful fun too. That said, there was the odd chance minigame, which was annoying. The boards really get the balance though, so no-one is unfairly treated.
The minigames are great fun, but many don't use the motion sensing, which was unusual. But not using it at all is better than abusing it like in Mario Party 8.
Though Mario Party 9 is awesome with friends, the same can't be said for single player. Solo (story) mode is actually, unusually, fun however, so single play is isn't as bad as in Party 8.
Additional to the main games, there are special minigame challenges that combine minigames with extra strategy. I found most of these to be quite lame apart from choice challenge.
Mario Party 9 is great with friends, an 8/10 for that. But single player included, it's not AS good. Still, with company, this is a must-have party game.