Qt certainly is one of the nicest GUI products around, and don't forget PerlQt http://perlqt.sourceforge.net/ which gives you the best of all worlds:

You can prototype in PerlQt and quickly convert to C++ if you need to - though on a 700Mhz Athlon (hardly cutting edge anymore :) I've had no need to since speed has not been an issue - you *might* suffer with resource usage though - Perl + PerlQt is a bit of a memory hog.

PerlQt is certainly good on Unices, and apparently fine on Win32 + Cygwin - watch out for licensing on Win32.

Finally, you get pretty good documentation and qt-designer. Yes, by all means learn how to code Qt by hand at first, but designer will save you a lot of time if the project grows. Designer is the overriding reason why I stick with Qt - not perfect, but far nicer to use than glade, or that commercial wx GUI designer (sorry, forgotten its name, I quickly gave up after test driving the trial download!)