Not bad at all! It forces you to re-think about your power plants every time every time you buy an upgrade, and the progress curve is good.
Pay attention to a technical issue, though. When you pause the main screen and change tab (e.g. Upgrade), returning to the main view you have the [Pause] button and the game stopped, thus you have to click on pause twice.
Also, it would be useful to have the tab bar always present, so that you don't have to return to the main screen when you pass from Upgrades to Research and so on.