Meh.
The upgrading seems a lil suspect however.
With the difference of power between one weapon and the next not being very much, you're better off using the first weapons and just upgrading them cause it's cheaper.
Also! You really should tell people that the primary gun isn't an instant shot gun, that is you have to press Space to start the crosshairs going then space again to actually fire.
That and the enemies get way too strong considering you're stationary and can't shoot down the planes :P When you see the Jet Carriers SHOOT IT!