I use designer when it's convenient. Some things, like toolbars and such, must be done programmatically (designer has no such functionality in v4), but I don't complain too much there - it's a simple matter. Other things, like truly dynamic menuing systems, obviously cannot be done through designer due to their dynamic nature (for example, a list of pushbuttons that gets populated based on what plugins the user has loaded).

Some things are possible to do in designer, but are very awkward. For example, creating a widget to insert into a bunch of other widgets (re-using functionality). Designer provides this functionality, but you must manually edit the .ui file to properly include it. Much easier to just design the thing via code, though you can of course design the shared portion using designer rather easily.

Of course, most simple widgets and dialogs benefit greatly from designer - being able to see exactly how everything gets placed, how certain modifications look in real-time, etc - it's a valuable resource.