I enjoyed this. But I do not usually like racing games.
I like the multiplayer feature. But if you make a sequel (And I hope you do) you should have a level editor and LIVE multiplayer, instead of just racing ghosts.
Uprgrades are your choice. If you do implement upgrades, multiplayer might not work so good because some players may be overpowered with upgrades. Instead use a level system to match people up with opponents the same level as them. I also think you should make the balls go faster, but that's just my own thought.
All in all, this is a great game and I hope to see a second one. (It's just too bad I quit because I suck >x<)