It's Contra basically, but it has a mode where you get exp and increase your health, power, etc. That was what we did, don't think we would've made it through Arcade. Gunvolt is a lot of fun, if you think you'd enjoy it I'm pretty sure you will.
People do what they want with their stuff. Some would say you don't deserve to play because you'll support Gamestop over the developers to save a bit money buying used.