We stayed in Rome for 4 days and it was too little time. Especially if you're staying at the heart of the City, like ourselves.
Marcella's place is just THE perfect place to stay when in Rome. It's literally (and I really mean literally) on a walking distance to EVERY main landmark in Rome! My wife and I had the time of our lives there.
One thing you should definitely do when you stay there is to get lost in the Roman maze-like alleyways and be awestruck when you turn on a corner and find yourself standing before one of the main city landmarks (did I mention they're ALL on a walking distance from Marcella's?). When you're hungry, get inside a trattoria on such alleyways and feast! Italian food is the best in the world!
If you like the nightlife, pizza del Fico (right downstairs) is the perfect place to go and meet new people. It thrives with people from all over the world. If you're not into nightlife (like myself and my wife), don't worry! Marcella has installed sound-proof windows, so you can sleep like a baby.
Also, Marcella is a wonderful host! She is very attentive, polite and careful. She's given us a lot of good advices when we got there and was always there to rescue us when we sent her a message with doubts (and we had a few of them... (Thanks for your kindness, Marcella)).
Two things are certain: we had too little time in Rome and we are coming back and staying at Marcella's again. No doubt!
The only thing which is not so good about the place is that there's no elevator. So, if you can't carry your luggage upstairs, you'll have some issues there. But only if you really CAN'T (as in MUSTN'T), because the place is well worth two flights of stairs!