I spent five nights in Ristorante Fiorentino in mid-May. Located within the walls of the old town, it is an easy walk to any of the museums and other sights.
Located up four flights of stairs, the accommodation is very comfortable and infinitely better value than the Albergo Fiorentino next door (just €5 in the difference) where friends of mine stayed.
The young lady who runs the establishment, along with her parents, works from morning to late night and is totally committed to keeping up standards.
Known to be the best restaurant in Sansepolcro, the food was excellent. The breakfasts consisted of all the deserts from the previous evening's dinner, along with yoghourts, fruit etc. They also served the best coffee I found anywhere in Italy. The dining room has a lovely ambiance: it is a wood-panelled room, with a distinct feel of 'old mediterranean' about it, with its large windows opening onto the streets below. While I was there they erected decking on the street, giving people the choice of eating outside at ground level, or inside at first floor level.
Great place and great stay!