Excellent game. Very good execution and use of the theme, with the only (very minor) problem I had was that selecting upgrades could be a little annoying with how stick the tooltip over them was.
Personally, I would prefer if only the title be displayed on hover, and then the full description show when clicked on, especially since you already have a different button within that menu dedicated to actually purchase the upgrade. But again, very very minor thing, and others might prefer differently.