I'd say it depends on whether or not you want the native Look and Feel of the OSes you're targeting for your application. Qt, like said earlier, is probably the easiest of the cross platform toolkits to use, however it is its own widget set. You don't get a native app look & feel unless you happen to be running on KDE.
I use wxWidgets at work. It can be frustrating at times and in some places not very polished, but it does give the native look and feel for the platforms you're targeting. It actually wraps the native UI controls to give them a common API on all platforms that wxWidgets is ported to.
