Well done!
The first thing I noticed is that all the controls are a part of the game. It's not "push A to unleash a sonic wave", instead I have to actually push the little bulb on the monsters stomach. Out of a cognitive perspective it's great. It makes me interact with the game instead of the keyboard. (even though the morph was activated with space)
As with all upgrade games it almost forces me to keep playing till I've upgraded the monster to max but that's fine since the game is fun to play and I just love snotting the crap out of those little spunge-bob soldiers. It is also well balanced; neither too easy nor impossible to beat. A game like this should not require too much effort yet still be challenging I think. That, you have acomplished :)
I loved the ending when the purpose of those UFO's are finally revealed and it made a rather classic and fun ending with the player encountering one last time all of the enemies found throughout the levels.
Great game all in all.
P.S. The reason you don't get a 10 is because I'm saving it for the truly revolutionary game. Yours was innovative and fun but not the kind of game that would make me go "Oh my GOD! That is fricking AWESOME!!"