It's good!
I like that someone made a shooter where we get power-ups rather than an upgrade shop. I can kind of understand the appeal of buying upgrades, but taking the micromanagement out so I can have pure shooter action is so much better. Also, using the power-up advancing track made me fan out a bit as a fan of Gradius.
It has issues though. It's poorly balanced, so it's very easy. The beginning is very easy, as would follow, but by the end of that level, you have so many power-ups that you're an unstoppable death machine. I cruised through the rest of the levels without much effort, and I'm not even talking about spreadshot and missiles. The levels are bland because it's just enemies and a backgrounds. I liked that the scrolling changed from level to level, but because the levels don't have any physical obstacles, it doesn't make that much of a difference. Having physical barriers and maybe some moving set pieces that you have to navigate would help on both fronts. Just don't go crazy with it. With the shooter, it's a thin line between fun and frustrating.
I liked it a lot. You have a good, fun starting point that could grow into something amazing.