Bring your earplugs, cause it may be noisy on the piazza Trilussa that is a few steps from the flat. The 4 nights we've slept there, there was a singer (with a mic and a guitar) on the piazza, and a big crowd singing and shouting. Despite the double windows, we could hear them, sometimes until 2 AM. If you are looking for a quiet stay, maybe this is not the best place.
Apart from that, everything was great! The flat was very clean and cool (thanks to the air conditioning), the terrace was nice and cosy (but there is no view on the river, a little disappointment), and there are many restaurants and bars around. Trestavere is very lively! Even the touristic attractions (Trevi Fountain, Colosseum) are easy to reach on foot.
The communication with Elena was easy, and her recommendations for restaurants were all excellent.