almost perfect
-add keyboard controls (space for fire)
-at least extend the invisible barrier to the halfway point
-add a few more varied upgrades like side shooting cannons and missles
-have your shield recharge slowly on it's own
-add a special attack for those extra sticky situations? something that deals a lot of damage and clears all the bullets on the screen
-make the diamonds one unique color so it's easier to pick them out. Maybe like make them all silver, and then make sure enemies only fire colored bullets?
-you should also be allowed to shoot down missiles. They're very irritating. Yes, I know you can use your shield against them, but i'd rather save the shield for more serious situations.
And finally a tip: In this game i have found the best strategy is to watch the area around your ship so you can focus on dodging and then just roughly aim with your perhipheral vision until there are only a few guys left.
I only made it to the level where you have to fight four of those boss dudes. I found it extremely irritating because the screen went all white and you hear an explosion every time HE is damaged. I thought that it was me dying when I wasn't even near any bullets.