Gameplay can be like a chore, but still rewarding.
I feel that this game has good rhythm and feel to it. I particularly enjoy the simple controls of it, making it easy for even the most casual of gamers to get into it. It also has some pretty solid physics supporting it, and I'm certain that the creation of even more levels would be a snap if need be.
I do think that a certain showcase of progress would greatly benefit the title. Tell the player how many levels remain before reaching the end of the game, and at the start, let them choose the level they want to play on (later levels must be unlocked by playing or passcode entry, something like that). It would also be nice to have a scoring system where it combines the time it took them to complete the game, and have other elements affect the final score like additional "treasure" pickups and the number of deaths the player has. So instead of an ending screen of "hope you enjoyed playing," it also gives the player the reminder of "here's how suckily you completed the game!" while throwing a bunch of numbers at them.
For some reason, players enjoy a final series of spontaneous numbers being thrown back at them. As it is, Rocket Car feels like a chore. Throw in some sort of tally, BAM, Rocket Car is a GAME.
I can even imagine players wanting to customize their cars as well. Just a silly little "custom vehicle" screen where the player can choose their car's color, shape, etc. Elements that don't affect gameplay per say, but allows the player to individualize their own gameplay experience.
You'll be surprised how simply throwing in a couple hours to a "custom car" screen will dramatically influence the score of a game. Because players looooove customizing. Almost as much as they love poptarts on rainy days. Almost. Not quite.
Everything else holds together well. Graphically, it's simple but has a solid color scheme that allows all the important elements to stick out. It flows nicely, looks sharp, and maintains a discernible sense of humor consistently.
This definitely was 30 minutes well spent. 5/5.