It was fun at the begining but with time its gets less interesting. It is too short to get boring what is a good thing. Last boss is too long - not too difficult too long, with full upgrades it takes a lot of time to kill him.
Full upgrade awards checks if you are full upgrade at the begining of run - it's a problem because I got fully upgraded in the same round I won and if I didn't restart the game I wouldn't get award.
Drill guys are too brutal, they are pretty much the only ones who dealt damage to me.