Way too loud. Impossible to sleep. The only quiet hours are from 3:00 AM to 7:00 AM. The rest of the time there are hundreds, if not thousands, of people walking by directly below the window. The cafes below the window don't close until around 1:00AM, at which point you hear them banging tables and chairs as they pack up. The traffic to and from the fountain, which is only about 20 meters away doesn't stop until 3:00AM. Even with earplugs in, my girlfriend and I could hardly get any sleep.
If you only have one day in Rome, it is fine I guess, but if you have a longer stay, pick a quieter location in Trastevere or the Jewish Ghetto. Cheaper prices for food and accommodation, plus they are still close enough to easily walk anywhere downtown.
The location of this place is unfortunate as the apartment itself is nice, clean, and comfortable. I wouldn't recommend this place to anybody, though, because it is just impossible to get any sleep or even feel relaxed.