I have a love/hate relationship regarding the AI opponents in this game. On one hand, I like that the computer offers a challenge. Getting 1st or 2nd place every race is difficult and it may take certain customizations before you win 1st place in a particular cup. The frustrating part is when you get bombarded by several other characters. It always seems to happen when you're on the home stretch. You've raced a perfect race, and on the last leg of the final lap, you get attacked multiple times in a row. You lose your item, momentum, and coins. By the time you get back up to speed, several karts have whizzed past and you cross the finish line at 5th or 6th place.
The old stages look great, with a lot of the updates to the retro stages. There are several characters and customizations. While the game does provide a challenge, I don't think it went overboard like the Wii or GBA. The controls are tighter, and you could win most circuits without using items and by simply taking the quickest, most efficient route. (assuming you were fortunate enough to avoid turtle shells and lightning bolts.)
All in all, great game. Great graphics, great controls, plenty of content, and a high replay value. The best Mario Kart game I've played since Mario Kart 64.