Alright, as you said, it is a prototype. Here are a few tips to make this a lot better.
Once you run out of coffee, you're screwed, right? There should be some way to recharge your meter. Because, As I'm typing this now, one little red block is spinning around my fat man. As amusing as it is to watch, it inevitably gets dull and denotes a low score. The buttons on upgrading did not work. Instead, the play button appeared and would not allow me to select my upgrade. When I hit play, it brought me back to the game and I had the same amount of coffee I had when the level was completed.
It's a shame that these glitches happened but, as you said, it is a prototype, and is, in a sense, still in it's beta testing stage. Since you're still working out the kinks, I've given you a modest 3 and a 4.
I do hope you can improve this game, as I would like to play the finished version to kill some time.