I liked this game a lot. It's the type of game we love to hate: something like "I won't play this anymore!... well, just one more try" hahaha
The only thing I didn't like, and the big one, was the controls. Aiming with the mouse where the direction comes from a character in movement like in this game is hard. I mean not in a good way. You have to shoot towards here, and quickly shift the aim to shoot there - but as the character travels on the screen and get passed on your mouse cursor, your'e not aiming anymore where you intended before, resulting in failure. And when the big challenge of the game becomes fighting against a punitive control scheme, that "anti rage-quit replay factor" vanishes. In fact, I gave up on the "Mastery" level.
One good alternative that I saw in other games with this mechanics - directional propulsion coming from character - is fixed direction control. Maybe some button that when pressed locks the character in aiming mode. Maybe even keep the mouse, but making the gun point to where in the screen the mouse is, not directly at the mouse cursor.
I could give a great example of a game that uses this mechanic and a nice way to implement, but since it's a mid-game upgrade, I don't want to spoil.
Overall, nicely done.
I only missed the Criti-Cola. And the game also needs a button to make Bob do something. xD
(but seriously, reference speaking, I would save the name "Marathon" for the last level *wink* )