Carmen's apartment is in a good location--walking distance from the Colosseum, and very close to a metro station from which you can get to the further sites (which are mostly within an hour long walk or less anyway).
Carmen is also a very hospitable host: she provided candies, snacks, water bottles, a bottle of wine, breakfast foods, etc. for us. We couldn't get through everything she had ready for us! Carmen wasn't able to meet us that day to give us the keys so we were instead greeted by her husband, who showed up exactly on time and was kind enough to give us a very detailed overview of Rome (where the sites were, which were worth seeing over others, which neighborhoods were safe and which weren't, etc.)
One thing that I should note is the apartment is particularly good for/geared toward couples--a lot of heart shaped and love themed art, furniture, decor, etc. I was just traveling with a friend so we weren't the best fit for it, but enjoyed staying there nonetheless!