My wife and I stayed 4 nights at Marcella's place, and loved every minute of it.
The apartment itself is very spacious and clean, with a fairly large kitchen. It's especially recommended if you're looking to experience Rome's nightlife- the apartment overlooks one of the hottest bars in the area, and the area is alive and full of tourists and locals every night. Fortunately, the double-glazed windows and air conditioning ensure that it's never too noisy inside when you're ready to call it a night.
Marcella herself was very accommodating and even allowed us to check in early, as well as arranging a very helpful transportation from the airport.
In short, the place is lovely and within walking distance to all the major attractions in the old city. I would wholeheartedly recommend it to anyone visiting Rome.