I stayed with M&B in Florence and it was a nice stay. The accommodation were as they were described. The location was convenient, close to the train station, and about a 15 minute walk to the old town. It was really nice to not have to rely on public transportation!
I did however find the bed to be slightly uncomfortable and the room to be hot at night. For the 3 nights we stayed it was a little on the pricey side and only offered laundry service for 5 euro.
All in all I would recommend this stay for its proximity but make sure to read the description carefully.