The apartment location was great - just outside Piazza Navona, which also provided quick access to a continuous taxi line if you want to leave the area.
As rooms in European cities and especially older buildings are usually small, we were pleasantly surprised at the size of the apartment. 
Unfortunately, when we arrived, we learned from the check-in representative who met us that the elevator was broken. Luckily we were all able-bodied and able to walk up the 100 stairs to the 5th floor apartment (note: apartment is on 4th floor, but in Europe, ground floor is considered "0" so it is physically on the 5th floor of the building), although with luggage and 100 temps, it was not pleasant.
This also made it inconvenient to "run down" for a coffee or gelato, which would have been nice given the proximity to so many great restaurants and shops.
I contacted the apartment manager who explained that they had called for a repair but the technician was waiting on a part. The repair was not completed by the time we left, and although not as bad as going up, carrying luggage down was not pleasant either.
I know it's not the fault of the apartment management that the elevator was broken, but if this is a deal-breaker (as it would have been had if we had elderly or otherwise compromised individuals in our group) I would suggest contacting the management before checking in to ensure the elevator is working.
The apartment was very clean, and the air conditioning worked well. One other thing to note, regarding accessibility, is that the second bedroom which is located in a loft area has a rather narrow spiral staircase that again should be taken into consideration depending on mobility of guests.
Overall, the apartment is a great value and our experience while in it was good.