good but
The game really starts to dwindle down after the mid game. There are few major issues with the economy that really make the game boring. Level 12 is the 'money round'. You'd think you start getting more cash as the levels get harder and you need to buy better stuff, but nope. You will be racking up solid 4k after level 12 where you can get up to 9k.
The decision to make the playing area bigger from lvl 19 onward was just bad. It takes longer for the enemies to get to you which even more reduces your chances of getting combos. I don't understand why you made the levels bigger :P.
The economy sucks because it's too heavily tied to the combos. Which means you need to kill a lot of enemies fast in order to get a lot of cash from the tougher enemies. With the levels having more tougher opponents, you take more time to kill them which means you aren't increasing your combo. This means lower income the tougher the enemies get. I see you are trying to throw some free kill waves at times, but it just isn't enough, and frankly those waves further add to the boringness of the game.
I suggest you increase the money you get from killing the tough enemies by a lot. This will ensure better economy the harder the enemies get and you don't need to throw those free kill waves which are just boring to deal with. I didn't finish this game as it was too boring in the last few levels. I was killing everything before they even reached my screen with the triple missile cannon. And as the income was so poor, like 4k per round, I wasn't going to save up for the beam weapon.
I suggest you take a good look at the difficulty curve of the game, as the beginning promises good game but it fails to deliver it further you get.