Broken game just reset my save for the second time today, this time when I was in the middle of the last cup.
It autosaves everywhere, even on the menus, so once it wipes your save, it autosaves over it instantly. So make sure you back up your save every couple races.
Edit - Got the Plat. Thankfully my save didn't get wiped again. Game is actually very fun and I enjoyed all of it except the glitches. The AI definitely weren't bad or cheap to me as posts above described. The AI on Normal and Expert difficulty are so easy that it's almost impossible to lose to them, and you can sometimes lap them. Even if they do manage to overtake, they just let you go ahead straight away. They only actually try to beat you on Extreme difficulty, but even then I won almost every extreme race on first attempt, except for the coin collecting one, that took 5 or 6 attempts. It does feel balanced to me because you get the same speed boosts as they do if you fall behind, so mistakes don't really matter unless it's in the last 30 seconds, and I didn't feel like the AI were attacking me any more than eachother.
The challenges were the difficult part in my opinion, theres 40, and some of them took quite a few attempts. One of the ones with inverted controls took a good couple hours to beat and I initially thought it might be a Plat blocker.
I'd say it's worth the price, fun game that not many get to experience. You can always just chuck it back on Ebay after and get your money back.