So, I got this on a whim and played pretty much straight through. It's not super long, but has a lot of fun stuff going on.
The graphics are great overall, though certain things like the Google font, zoom slider at end of level, and SD cards feel really tossed in. Yeah, Unity engine, whatever, but those things are distractingly out of place.
The controls are fantastic.
The action is pretty simple, but feels solid. Shoot and your ammo refills slowly. You can buy more successive shots (longer ammo gauge) in the store. every boss you beat gives you a new weapon.
So, it's basically a mix of Megaman and Mario. You may catch references to Bullet Bills or have other nostalgic flashbacks like I did.
Overall short and fun, but no replay value. Still worth the download though.