I love running over those pesky aliens. Very satisfying screen shake, crunch sound effect, and splatter graphics! Reminds me a bit of the original Grand Theft Auto.
Well-paced action, smooth controls, rocking soundtrack. I also love the front end of the game--aliens don't harass you until you run the first one over, and that's how the game begins.
One thing that I found a little confusing is the health bar. It depletes automatically, so presumably it is a joint damage/fuel meter. I think that making these separate meters would make more thematic sense, and would give the player a little more to do (picking up fuel barrels and whatnot).
Also, a bit of enemy variety would be good too.