I use Designer for QMainWindow and QDialogs. I find it handy to get the layout right, especially when I want to stretch "correctly", whatever that is depending on the dialog. It helps to connect simple signals/slots such as pressing enter in a text edit box to activate the Apply button. It makes it very easy to write code with the automatic slot naming, such as on_QSpinBox_valueChanged(int ID).

The times I write code are when the contents change.. such as a joystick dialog where it loads sliders for each available axis, and buttons for each available button. A library handles most of the work and gives me counts for each.