It really helps that I was sick as a dog when playing this because I didn't mind grinding since I can't really doing anything else. I did like the different enemies, and this would have been a good paced game if:
1.) as a lot of people mentioned, having an experience and money factor makes the game start really slow, and the player may lose interest.
2.) once you buy multiple cars and level up a few things, the game is over in a few moments.
3.) you need to change the recharge time on invincibility since you truly become invincible when all three cars have it upgraded at least once.
4.) to that point, the rocket launcher car really doesn't really do anything special, but having two machine gun cars and one plasma car is too powerful.
I love games like this, and I did enjoy the enemy mechanics and how many different enemies there were, but I feel as though games like this should take around 30-40 minutes to beat or else you risk the player losing interest. I thoroughly enjoyed it though; I would just hope you would make some tweaks if you ever make a sequel.