FLTK is extremely lightweight and easy to use, and quite fast to boot. It doesn't integrate very well with e.g. platform accessibility hooks and stuff like that (but then, not many toolkits do). Highly recommended if you're using C++.
FLTK's major problem is that it is too much of a C++ system; while wrappers for some languages exist, a lot of the properties that make it desirable in C++ are lost through those wrappers.
The language itself also has some serious race conditions that are impossible to debug unless you're on really slow/embedded hardware. For example, I used the Dillo web browser (which is FLTK-based) for a couple years when I was stuck on an 800MHz PC, and when the system was swapping heavily, doing things like opening the (FLTK-generated) file-save dialog or dragging a window scrollbar (IIRC) would lock the mouse - FLTK uses X11 cursor locks to constrain the cursor in some situations, and due to the system's heavy I/O load, FLTK wasn't releasing the locks, and I had to kill the entire browser.
Hmm. Good point; AFAIK Dillo doesn't use threading.
I'm honestly not sure what was causing it, but I know that it only happened when the system was swapping a lot. So something that expected something else to be done or take a short amount of time didn't.
For all I know it could be the way FLTK is using Xlib (I don't think it uses XCB yet).
