This is the sort of game that reminds me of Micro Machines, back in the 90s, when i was a kid. The graphics are very similar to it and the only thing you've really changed is the characters you can play as.
The gameplay is good, and the AI can be quite a handful in the harder modes. I like the way that you've got the cups available on easy medium and hard, just so you can let people of all ages and abilities play to win.
I'd have said that to unlock characters, you need to win race series in any difficulty mode, rather than just easy, as some more advanced players could get quite bored with this game playing through the easy levels to unlock stuff.
I'd also suggest a tutorial level, where you go to driving school and learn about the basic weapons and how they affect the race. Mention that each character has a unique weapon that does something different, so that it gives the player the inspiration to go on and find out exactly what is meant :P
I look forward to seeing the next incarnation. I would like to see more levels, maybe a desert one and indeed possibly an underwater or space level as well.
[Review Request Club]