I use wxWidgets. Most of my experience is with version 2.8.
If you care deeply about making a native applcation that truly has a native GUI on Windows, Mac and Linux, wxWidgets is great. Nothing else even comes close. Java, QT, XUL, FLTK, TCL/KT and others all produce programs that aren't quite right on some plaforms.
The truth is wxWidgets is pretty much its own system, an SDK in itself. It has a tremendous amount of somewhat complex design, like sizers, which means you have to go to some extra effort. Of course, for making things work well on all platforms... not simply just work, and not work well on Windows but end up sub-standard on Mac or Linux, but work truly well on all 3, the extra effort to use wxWidgets is definitely worthwhile.
But the truth is you do have to put in extra effort. wxWidgets has great documentation to help, but the other truth is everything is heavily steeped in C++ class heirarchy. If you're good with C++, it'll feel pretty natural. If not, well, you'll get much better with C++ in the process, if you persevere. In the end, if your goal was a native application that truly works natively on all 3 platforms, the sort of thing users take for granted and never notice, you'll be rewarded.
But if you don't really, truly, earnestly care about targeting all 3, if only Windows has to be high quality and the others are afterthoughts, or if you just want to get things done as quickly as possible with minimal learning, you'll probably find wxWidgets to be far too much trouble.