I stayed at Hostel Santa Monaca for just 1 night as I didn’t have much time to spend in Florence.  It’s a large hostel in a beautiful old building with an interesting history.  The reception was manned by older staff, who didn’t seem to engage with their guests with any joy, although they did ask if I wanted suggestions on what to do in Florence.  
I stayed in a 6-bed female-only room, which had a ceiling fan and portable fan, and was clean, but did feel a little cramped.  All the power points were by the door, rather than close to your bed, which wasn’t ideal.   There was a toilet nearby, and the showers were a little walk away.  There was a good-sized kitchen, although I didn’t use this, and a big dining area.  It was about 20 mins walk from the train station, with a supermarket next door.