Well, my experience is that there is no such thing like 100% portability as in meaning: use the very same code everywhere.
I tried with Java plus SWT and now with Qt but sooner or later you'll discover aspects which need to be taken care of differently depending on the platform/OS (whatever you call it).
Of course this highly depends on your application and what you are doing with it but "higher level frameworks" such as Qt help you keeping a very high percentage of platform independent code.

By the way, Qt is much more then "just" a GUI solution, it really aids in developing platform independent code in many many fields.

Edit:
By the way, I am my best customer when it comes to developed applications.