If there are powerups, the population of powerup bubbles is very low. This is an interesting concept for a game, but I think it needs some tweaking to make it more interesting.
Although the game designer says that they have persistant upgrades, this simply isn't the case. Frankly what would be a better mechanic is to have upgrades BE persistant and for them to cost more - this gives two incentives to replay old levels.
+ If you agreee!
Okay - the hidden charachter the "time traveler" can be unlocked when you take Boffin and the dog to the car park. Hints on beating bugzilla: Take the Scientist and Veteran with you to give you +20 to your attack, use bug spray to reduce bugzillas damage (temporarily), and use the alien gun (highest damage).