I preordered the game and got it the day after its release date (thanks to work stopping me from playing it the day of the release date).

I've triple starred all 32 cups, defeated all 32 Time Trial Ghosts, and unlocked all parts except for the Gold Glider (10,000 coins will take quite a while to reach). I actually found that mediumweights/cruiserweights (like Mario, Luigi, Donkey Kong, and Rosalina) with high top speed parts (like the Circuit Special and the Slick Wheels) are really good for 150cc and Mirror (though you'll need to practice drifting and mini turbos a lot) since the top speed will get you so far you can get a huge lead in first and be able to avoid the onslaught of items the AI will throw at you (though you'll want a shell or banana in stock to protect against red shells just in case). I get so far ahead, that sometimes even blue shell hits aren't enough to get the 2nd place person in sight of my rear view mirror.

Also, feel free to add me and challenge me online. I'm Player¼/Fidesio.