We liked staying at Elena's place. Elena upgraded us to a larger unit in the same building, which was much appreciated. Moreover, since our train was leaving Rome in the late afternoon, Elena allowed us to check out at 3:15 at no extra cost, which gave an additional few hours for sightseeing without having to find a place to store our luggage. Thanks again!
As others have pointed out, the location is fantastic. We walked everywhere, from the Borghese Gallery to the Vatican to Palatine Hill. You are in the middle of historic Rome, with interesting streets and fantastic restaurants all around. There is also a Coop supermarket practically next door, where you can buy provisions for eating in -- which is great for saving money. The apartment also comes with some very welcome extras -- the courtesy umbrellas in particular were very much appreciated on the first day of our visit.
The apartment we got (which is not the same one as in this ad, but presumably similar) was clean and comfortable, with very reliable Wifi.
As others have mentioned, the bathroom is rather small, the hot water sometimes runs out in the evenings temporarily, and the apartment is rather noisy due primarily to noise that carries from the street. Even though the street has little automotive traffic, pedestrian traffic is heavy, noise bounces off the walls, and the echoes of conversations of people walking are surprisingly loud even until very late at night (when the bar crowd stumbles home). Even though our bedroom did not face the street, we still required earplugs to sleep.
Note that Google will tell you that the fastest way from the airport to Elena's apartment is to take the train to Trastavere Station and then the No 8 Tram. However, if you are not familiar with Rome's streets and transit system, it's much easier to go to the main station (Termini) and then take the No 40 Bus. Elena will give you detailed directions, and if you get mixed up, there is ample English signage and tourist assistance available at Termini to point you in the right direction.