I've used WxWidgets before (C++). It's easy and fun. Afaik, a significant amount of UI design is done with design editors or various XML/LUA importers, so the toolkit you use essentially doesn't really matter, because the event-driven code you will write will be very similar for almost everything.
The WxWidgets tutorials are great and the forums are helpful. If you're wondering how WxWidgets compares to other Toolkits, they have a great wiki page dedicated to it
