I stayed at Sonya's place for 2.5 months. The apartment was nice and quite spacious compared to what I was used to seeing in Paris before. It's located very close to the Belleville metro as well as several restaurants and grocery stores such as Franprix, Naturalia, and Monoprix.
I didn't have any problems while there. The apartment faces a courtyard so it was almost always quiet no matter what was going on outside (there's a weekly outdoor market but I often didn't notice until I went outside). There are several bakeries nearby so you'll often wake up to the smell of fresh baked bread coming through the windows. And there's a Neoness gym two doors down that I used almost daily.
The only downside for me personally was the lack of an oven -- I didn't realize how much I baked until I didn't have one! But this was not a major issue, as I was able to cook all the meals I wanted on the stovetop.
Belleville is a bit removed from all the touristy areas of the city, which was nice for me since the neighborhood was quieter. It can still get crowded on the streets since the sidewalks are thin, especially on market day and Friday evenings given all the restaurants along Rue de Belleville. People also tend to loiter on the streets, but I think that's just a Parisian thing, haha. It was nothing overly concerning.
Sonya's daughter Lucy facilitated the check-in and check-out process and was an absolute joy to work with. Very quick to respond to texts and an all-around sweet and helpful person. Overall, I would stay at Sonya's place again!