FLTK isn't as rich (no system abstractions, etc) as wxWidgets or Qt, the main players in the cross-platform GUI toolkit arena, but it is very straightforward to use.
Last time I used FLTK it seemed alright but the windows and widgets looked like absolute garbage. Looking at their webpage and the screenshots now, it seems like that might not be the case any longer.
Are they using native widgets yet? Or are they still doing custom drawn widgets?
I'm not sure about the version under development, but the current release still uses custom drawn widgets. Not so pretty, I suppose, but ideal (to me) for cross-platform utitilies.