Graphic is great, so is the gameplay. The idea of buying ships or/and upgrading weapons/engines/special weapons is good in an arcade game, it gives the genre a some new perspective. Also that you need to buy ammo and repair is nice. Each episode is a following story of the previous you completed so you actually get a feeling of a storyline. It's almost like Starfighter (if you tried that), but much better in every aspect.
Also it's easy to make your own episodes I read somewhere.