I'd rate it an 8. I was so completely uninterested in this game when it was first announced and free, but hey, it's free so BOOM, downloaded, played, addicted.
Like has been mentioned there should be more levels, and I think more ships and with the ability to paint your ship whatever colors you want and maybe even name it so you get a bit of ownership of your ship (see what I did there).
Overall I'm impressed and it turned me from completely uninterested into "I actually like this game"
And it works well with the Vita remote play, but the analog sticks on the Vita weren't designed for the precision required for Resogun so don't expect any great scores. Also, the boost and overdrive are mapped to the L1/R1 on the DS4 which means you have to use the touchpad on the back of the Vita far too often when they should have made the "Throw Human" and "Bomb" as L1/R1.