Mario Kart 64 is a good game to help you remember the good ol' days, but it feels somewhat dull by today's standards.
It's not all bad though, the game still holds its title as an awesome multiplayer experience. You'll find yourself blowing away two hours of your Saturday afternoon playing battle mode with friends. Another positive in MK64 is the course design. MK64 has some of the best courses in the series. Some of the courses, however, are extremely long (7 minute Rainbow Road course. Yikes.) and get repetitive.
If you played MK64 ten years ago and don't own it anymore, it might be a good idea to invest in the VC version for $10. If you already have the game for your N64, there's no need to download the game for your Wii. MK64 is a decent racing game and a fantastic trip to the good ol' 64-bit days, but the game just isn't what it once was.