I use the Designer. I avoided it in Qt3, but changed my mind in Qt4. The Qt3 Designer created widgets, which meant I was always subclassing an autogenerated class. But with the new UI form classes this is not a problem. I can treat the UI as a true object, and use it the way I want to use it. I typically use the UI as a private data member, but sometimes use multiple inheritance instead. The second problem with Qt3 was that it generated "messy" code (imho). But Qt4 Designer generates very clean sensible code.

I did have one GUI a couple of months ago that was too complicated for Designer. I needed to use a couple of custom layout managers in it, and some "hand tweaking", to make lay it out correctly. But that's the exception.

I don't put everything in a single UI file, though. I tend to split it up. The main window, menus and toolbars will be one UI file, and the central widget antoher UI file. If I have a tabbed widget, then every page will be a separate UI file. Etc.

Designer has proven to be a huge timesaver.