I've never used FLTK myself, so I couldn't comment on that. But wxWidgets is pretty straightforward to use. In the past I have ported/converted a couple of C++/MFC based applications on Windows to use wxWidgets and made additional changes to the codebase to allow the program to build on multiple platforms and compilers. So from experience, wxWidgets was relatively painless to use.
QT is another rather good GUI toolkit. I've not used it too much so far, but I'm gradually getting the hang of it!
You know, I'm quiet imprssed with FLTK. It's not as smooth to look at but if you get creative with it you can do allot with it rather easily from what I've seen.
Its also very light and small which is what I'm looking for. wxWidgets is fantastic but creates a very large file which is making me look around for alternatives. I've actually made half a decision to go with FLTK as long as I can do what I need with the widgets.
You can shape the widgets allot better, curves etc and even though wiki says there's less widgets in fltk I'm not sure about that. Seems to be everything there plus a few extras.
I was also looking for a ListBox but found that FLTK has a table widget which is like a spreadsheet but a little more functional. You can manipulate it to act like a listbox and put in and expand as many tables as you like. (this is almost sounding like a Christine review)
If anyone else would like to comment on FLTK please do.. I'm interested to know what others think about it.