Oh well, I have to throw my 2 cents in. :-) In my case, it's a choice between Perl/Tk and Perl/Gtk2. I never got into Wx because it's a bear to install and it's too OO for my tastes. Perl/Qt is old has isn't being actively updated.

Tk is easy to learn and fun. Gtk2 is more difficult, because it gives you more control over everything, from the signals to the underlying Gdk windows.... but that complexity can cause alot of trouble until you figure it out. BUT once you do figure it out, man can you do cool things in Gtk2, like custom-shaped windows.

Gtk2 is actively being developed with the c-libs having bug-fix updates released monthly, and the Perl modules following shortly. That inspires me, and makes me see a bright future there. On the other hand, Tk is like an old-but-solid-toy..... it's still there, easy to play with, but it dosn't quite match up with the glitz and new features of the Gtk2 toy. For example, Gtk2 labels allow you to use pango-markup strings(mixed color and font size), where Tk only allows simple strings. On the flip side, you can't bind to a Gtk2 label, without an event box underneath it. Tk lets you bind to the label.

The one real advantage of Tk, IMHO is the Canvas(and it's steroid-pumped brother Zinc). They both exceed the Gtk2 Gnome2::Canvas (which has lost development status). I think Canvases are where the action is. You can make anything on it, and it runs well. I will mention that replacements for Gnome2::Canvas are in the works.... so the future may be bright there too.

Finally, one other advantage to Perl/Gtk2, is that once you get something running, you can port it to the C version of Gtk2, for a faster compiled version (assuming you have that skill ).