I agree to, Designer is a great help.
Try to do simple task like changing color of the Ui elements with QStyles without Desiger. In that case I would need to recompile my code each time to see the changes, with designer I can see everything right away.
Said that I personally use Designer only to design my Ui elements, rest of the code is in the program, I mean here especially signals, etc...
That way I can redesigned my GUI fast (if that's needed), and that's most important to me, especially with my one core old CPU. This also helps to keep my GUI code separate from the app code.