It depends on the situation, really. I think both have their place, and I usually use a hybrid approach.
There are always some things that the GUI builder can't do (for example setting the color to a UIColor constant in Interface Builder). On the other hand, most of UI work is pretty mundane (for example adding static labels).
I prefer to do the mundane stuff in the GUI builder, and the more interesting stuff in code.
Also, I occasionally find myself editing the XML produced by the GUI builder (Interface Builder and glade in my case) by hand.
