I'm now on the Qt wave, but had to use wxWidgets, .NET Windows Forms, MFC and few others. Never been too proficient in them, just enough to support and add features.
So far having the best experience with Qt - things like styleSheets and the data models (table, tree, etc.) are th best things I've seen. (I'm not much familiar with CSS, but somehow I'm getting exposed to the concept through qt-ones).
With .NET Forms it always seemed like I hack to go low-level and do some Win32 GUI stuff (which for some strange reason i love - since I do love "C" at heart).
I've also seen WPF, Reactor, etc. - and I simply could not understand them - either they look too foreign for me, or I don't know.
As for non-C/C++ ones - I loved LispWorks CAPI - it mimicks wxWidgets that it wraps native controls, but it's so easy to write bulk UI stuff in it (for me)
