Not bad, needs work.
I wouldn't say it's BAD, but it really needs some tweaking, in my opinion. The most common argument I've read so far is that it's SLOW.
And good god, it is SLOW. I coped with it for about 15 levels. The first 4 or 5 levels were ok, but after that, there is no possible way to get a decent stream of money, because the tanks get harder to kill. The timer is just too short to finish off sufficient tanks to make any money. so that left me frustrated, since there were a few tanks that only were one shot away of killing, but no time left.
How about a little money bonus for each hit, and a full bonus for a kill? Or perhaps giving a bonus after finishing a level...?
Good luck and hope to see more of this!