Martina's apartment is an excellent place to stay. We liked the lounge area for relaxing in after our tours (one was 5 hours long) and after shopping trips. The bedrooms are all very nice, with plenty of bedding and pillows available. An en-suite and a house bathroom made it easier for the 4 of us to get ready to go out in the morning.
The location is a 20 minute walk to the Colosseum, 15-20 minute walk to the Pantheon, Spanish Steps and Trevi Fountain. About 10 minute walk to Capuchin's Crypt which is also a good place to see, especially if, as we did, you get the tour that includes the Gregorian chant. Several shopping districts are anything from 5 minutes to 20 minutes away. The Vatican City is easy to get to, just go to the nearby Metro station (1 minute walk) and it's a 15 minute ride away.
Martina was very helpful, organizing our airport transfer with a very nice driver and his minibus, also Martina helped us with a key drop for one of our guests that was arriving while we were on a tour.