I've been there in August 2008. A place really worth to visit (although finding accomodation in the town is somehow difficult) hosting valuable historical remains but also home to a beautiful natural landscape. I traveled there from Sivas by the morning train along a breathtaking river valley and I came back the same day by the local minibus in the late afternoon. The six hours that we spent there were quite enough to visit the Great Mosque & Hospital Complex (UNESCO World Heritage), to climb to the Kale (Citadel) Hill, to walk through the picturesque narrow streets of the city centre and to have a brief lunch but were insufficient to visit all the historical monuments (mosques, tombes, old houses) spread all around. I would reccomend a 1-2 full days visit.