I'm just gonna say, Mario Party 9 is not as good as it seems. At best you should only rent it or borrow it from a friend or something. =P
Mini-games are AWESOME. I'll give it that. The controls are good, there's only a FEW luck games (though there's some boss battles that are based on luck which can screw you over >__>) and the design of them is amazing. HOWEVER, in Party Mode, the mini-game payouts... are just too close. 1st gets 5 mini-stars, 2nd gets 3, 3rd gets 2 and last gets 1. Battle games are for ONLY 3 FUCKING MINI-STARS from everyone, and the payout is 6 for 1st, 4 for 2nd, 2 for 3rd, and 0 for last. Oh hey, this is only 1 FUCKING MINI-STAR more awarded for 1st and 2nd AND last gets nothing! That's a total game changer! [obviously sarcasm

] But anyways...
Everyone moving at once... ugh. More luck, luck and luck. You have like NO control, and dice blocks are not very helpful. I want an item system that's GUARANTEED to help me, not A CHANCE TO HELP ME. When you use the 1-2-3 Dice Block and you need a 1, you get a 3. When you use the 4-5-6 Dice Block and you need a 6, you get a 4. When you use the 0-1 Dice Block, when you need the 0, you get a 1 and then get screwed over. Captain Events are also pretty luck-based. The Captain has an advantage over the others for each one... All you can do is pray.
The ending can EASILY screw you over just because of a FUCKING UNLUCKY DICE ROLL. The ending has several Bowser Spaces... If you're the Captain and you land on Bowser, you better pray you don't get "Lose half your mini-stars!". Otherwise, you're screwed. Also the Boss Battle game is worth a lot more mini-stars than usual, so make sure you do win that. =P There's also a bonus for being the Captain (50% more mini-stars) during the Boss Battle game, no matter what place you get.
In the end, the classic mode was better. You have more freedom, more replay value, more fun, less luck... If they do bring the Car Mode back, they should make it optional at least AND make it a LOT LESS luck-based. I'd love an actual item system, and boards with unique, fun objectives. =P
...I didn't intend to type this much, but I hope this helps you with your decision of getting Mario Party 9 or not.

(It may seem rather negative, however, it IS the truth.)