Location location location! Within 10 minutes walking distance, you're either at the Piazza Santa Maria in Trastevere, the Campo dei Fiori, Gianicolo or the Vatican. Colosseo and Piazza del Popolo-area are a bit further away but totally do-able by foot (we travelled with our 4-year old son and didn't bother using public transportation at all - we did use some gelato stops as bribery though).
The apartment itself was clean, comfortable and nicely decorated. Can't say anything about the kitchen since we didn't eat in once (you'll find plenty of nice restaurants and bars just minutes away in central Trastevere), but it seemed to be all right.
I would recommend to use their taxi service, a driver was waiting for us at Fiumicino when we arrived and picked us up at 04.15 AM sharp. You'll pay just a few euros more than you would pay for a regular taxi but it will safe you some hassle.
If you're a light sleeper, you might be bothered by the traffic on Lungotevere (the apartment is on a lower-lying street next to Lungotevere). Closing the windows helps enormously but it will not entirely block the sound of Roman traffic.