I have done both and I have found the designer to be a great help, as soon as you work out all the quirks with it. First off, (your likely already doing this) but I always devire the classes that they make with designer, otherwise any code you place in the source file is gone after the moc. I found that you need to sometimes place the includes in the source file that will call your objects for the forms otherwise you can run into errors where it can't find the headers for the widgets (of course you need to include the form header files as well). Another thing is if you start getting crazy errors on a compile, clean out the project. This will remove object files and such. Sometimes it gets confused and doesn't compile files when it needs too.
My first Qt project were all done with out a designer. I find the designer a nice compromise because you can get the size and position etc.. set quickly and still have control over the code.
Thats my impression of the package, I enjoy it. My classmate and others that I know from a Visual C++ or VB world despise it.