The game was pretty decent for what it is. My only issue were enemies shooting from "beneath" the screen, where I can't return fire. I would often get hit from shots that I just couldn't react to. As an Alpha, though, it was fine.
Shooting mechanics were good, though I would have cannon upgrades also improve damage to the HQs. At that point the map is usually cleared of enemies anyway, so I'm just clicking 20 times at an 'opponent' that doesn't move nor shoot back.