I agree with Kingwayner that it would have been nice to be able to upgrade ones original gun. Besides that I thought that barriers were a bit expensive when consider the path movement of the enemies. Their path wasn't random enough for them to be useful.
I got up to wave 23 with score 61645. I'm assuming that's not very far although it would have been sweet with a global high score.
Great graphical style, very easy to play, it had a good feel to it and the progress of wave difficulty was great. 4/5