First of all, it's clear you've put a ton of work into this game. There are a lot of features even outside of the core game loop that beef the game up in positive ways. As for the ads -- you're right, they aren't intrusive or annoying.

As for the core game loop, I hope not to offend when I say it doesn't really do anything that hasn't been done before, but I don't think that's a bad thing or something you didn't already know. My problem with the game is that it is pretty repetitive. I only played a few levels so maybe I'm missing out on new mechanics that get introduced later on, but the game seems to follow the pattern of hanging low and shooting anything in your path. There are moments where your ship gets upgraded to do some pretty impressive stuff, but without proper explanation I was unsure of how to get to these moments in the game. My suggestion would be to add new enemies that interact with you differently, and to explain how the ship gets the in game power-ups. I'd also make them permanent for the level -- so if the player goes out of their way to do a certain thing they get the power up until the level ends.

To end on a positive note, I really liked the artwork. Some enemies had colors that I don't think quite fit in with the space theme, but besides that the game was awesome to look at!