I absolutely love this game! I did find some bugs which I will try to explain in this review.
I somehow managed to open one of those garages. When I played the next run the garage seemed closed but I could still enter it.
When I tried a new run the items I picked up stayed away (except for the powerups), unless I restarted the game with G. This might have been intentional, but it would be nice if at least the energy packs respawned for a new game (and maybe also the obstacles).
Beating the rivals seems... odd. Sometimes I feel like I never passed the rival and I still win (never mind, they probably crashed), some other time I clearly beat the rival and it said I lost. At one point I also saw a rival drive off the road.
After pressing G the achievements seem to reset. I beat two rivals in a row after a reset, yet it said I beat 3 rivals (I did beat the other one before pressing G, maybe it stayed unlocked).
I don't really get the car unlocks, when starting the game I can only pick the red one. When I find a super teleport I can choose any car I like without unlocking it.
When on the highway I paused the game and the other cars kept going.
The music doesn't seem to loop if you spend enough time in a level.
At one point a super teleport (the volcano caverns) kept crashing the car at spawn, and the car kept spawning a bit further. This problem stayed in the next run so I had to reset with G.
Try sliding into the highway teleport, the car will be sideways. I was able to jump off the highway like this, but when entering the next level the spawn bug from above happened again.
Some things should probably be explained in a help menu/tutorial, like the 'switches'. First I thought they were security boots or something.
The music and graphics are great! I absolutely love the whole retro/80's vibe.
There's a ton of things to discover, which is great.
The highway was also a nice touch, finding my way in this game was a lot of fun.
The controls are hard to get used to, but fair enough for the game.
Overall it's an amazing game. There's a bunch of bugs but I guess that's what you mean by '90% done'. It's still very playable and a lot of fun. I hope to see more of this.