Casper's apartment was a great place to stay. It is in a cool, hip neighborhood that definitely makes you feel more Parisian than tourist. You get an authentic feel of the city. The area felt completely safe for us day and night (two adults and one 4 year old coming from a larger metro area in the United States). We were typical out until 10pm or so and never felt uneasy. Noise was typical large city apartment noise: cars, mopeds, occasional sirens, doors opening/closing, people walking around in adjacent apartments. We had no problem with the noise, even when the apartment across the hall had people over for a dinner party.
The apartment itself was very nice for our family. The child bed in the second bedroom was great for our daughter. Was perfect to have our own spaces to sleep, especially if our daughter went to bed before we did. We could close the door and still hang out in the kitchen or other bedroom. Beds were very comfortable. The kitchen was well stocked with with everything that you would need...a toaster, french press, pans, dishes, glasses, utensils, etc. The bathroom was top notch for a European apartment. The shower is amazing.
The recommendations for restaurants, parks, etc from Casper are great too! Don't skip on the bobun recommendation. The apartment is incredibly close to many restaurants, grocery stores, and bars. It is also one block from a metro station that connects super easily to every area of Paris.
Communication with Casper was great. He was very quick to answer questions and check in frequently during our stay. Casper and his family were on holiday when we stayed so we never met, but check in was a breeze. He gave us clear directions of how to gain entry.
One minor criticism was that the Wi-Fi speed would get very slow during peak usage times (after 6pm or so). It still worked fine to look things up or post pictures, but was slow or would disconnect every so often. It was difficult to stream video during these times. Not a huge deal, but worth noting.
Overall: If you want to stay at a nice apartment in an authentic Parisian neighborhood, book Casper's place. We enjoyed our stay and would recommend it to others.