It completly depends on the size and needs for a GUI.
Qt is a good solution, if you don't want elements overlay the rendering section (e.g. no in game GUI). If you have just a few buttons then you most likely will write a small class on your own, otherwise if you want to use quite a few diffrent things (labels, buttons, slider, windows, etc.) then you most likly want to take a look at librarys explcitly build for SFML, like
SFGUI or
TGUI, or use some more generic libraries like
libRocket or
GWEN, for which you'll have to write your own little SFML renderer (except GWEN has one already).