6 of us stayed here over the six nations weekend. The flat itself is probably more suitable to 4 people, but as we were only there for sleeping it was fine. Communication with Marie before and during our stay was great - super quick responses and very helpful.
Checkin and check out was extremely easy and we had no issues.
You can walk to and from Gare du Nord but just be prepared for the small alpine climb as you climb through Montmarte.
Overall, I'd recommend this flat, it was clean, modern and we'll equipped for people to stay a few nights.