Some time ago since I used Delphi, but in my opinion the Qt designer isn't actually very good. Recent Delphi designers and all Windows Forms and WPF designers are far more powerful, though I don't know how the Delphi designers of today
handle form layouts. The Qt designer has so many limitations and I never found it a pleasure to use it, even with the latest version integrated into Visual Studio. It's not easy to get custom components to work in the Qt designer, despite Qt forcing you to derive many controls if you want to change the behaviour. Today, I only use the Qt designer to provide a very basic layout for the forms and create everything else in code.
On the other hand, I don't know what you would like to do with a Qt designer in Delphi, since Qt isn't available for Delphi and many properties and methods do not have direct equivalents in the
VCL or in Windows Forms. But if you just want to use the form designer, use it: The ui files are in
XML format and can be easily transformed into Delphi code. Well, at least if you have equivalents for all the controls and you don't run into problems with the differences between Qt's signal/slot-system and the Delphi event mechanism.