I stayed for almost a week at Camere del Cavaliere. Expedia offered me this hotel one week before my trip; I had booked elsewhere, but they told me that the other place couldn't honour the booking; I accepted a second place, which also cancelled on me, and so had to take this place due to the short notice.
The hotel appears to be in a converted apartment, with four bedrooms and no common areas. Check-in was easy: although there is no check-in desk, the staff contacted me by text message, and came to let me in when I arrived. Note that the lack of a reception means you can't leave your luggage after checking out if you're leaving town later.
The hotel is very conveniently located about 10 mins from the Vatican and 5 mins from Ottaviano metro station to get to the centre. There are a number of restaurants and cafes nearby, most of which are cheaper than places downtown. Staff were helpful (no communication difficulties), and the room was kept clean.
However, there were several weak points. First, there is not enough hot water. Each room has its own water heater, but these only supply enough for a 5 minute shower, and take ages to heat up. This was my main gripe. Second, breakfast consisted of two bread rolls per person, and access to the coffee machine. Don't expect a hearty breakfast before your sightseeing! Some yoghurt, cold meats etc might help. Third, the pillows were too soft and thin, and only one per guest.
Overall, the place was decent enough but not outstanding.
