PROS
--The game is very fun and the variety of enemies is very good.
--The colors are very nice and the simple graphics give it a certain charm.
--The sounds of the game are great and give an excellent feeling of impact.
CONS
--The game economy often gives you two options: buy something bad or buy nothing. (I suggest you increase the amount of coins)
--The item collection area is too small. forcing you to stop and aim at it in order to collect it (just make it bigger like X2)
--Most upgrades are not worth buying as they have more downsides than upsides.
--The laiser weapon is horrible, (It would be much better for you to start with it and have better weapons to buy, (instead of taking the risk of buying a bad weapon and dooming your game)
CONCLUSION:
(All I said is my honest opinion, I'm not trying to put your game down, but just giving you tips on how it should improve)
it's a very promising game, with these fixes and the implementation of "achievements" I can see myself playing several hours of it. Good luck