Good game but...
I found a bug with the life buying system: when you don't have enough money and you buy life, the price increases, even though you didn't get the extra life. Also, when you DO buy the life, your money isn't subtracted (it stays the same as it was before).
Also, I found this game a bit easy. I got up to about level 60 without much of a problem...the trick is to keep pressing 'z' and clicking -- you end up shooting normal shots and freeze shots alternately, which is like an instant kill for everything :p. I don't know how you'd correct that though. Maybe add a time delay in switching between weapons?