Stayed here 1 night in Paris with three family members & we all loved Sophie's flat! it's a fourth floor flat with the Gare du Nord essentially right outside the doors, you can even see it from the window.
We never met Sophie herself, but her friend Baptiste checked us in and the check-in went quite smoothly. The flat looked just like the pictures and had all the essentials you could need. Just a note: the flat is up 4 floors with no lift so that's just something to keep in mind if you're bringing heavy luggage.
I would highly recommend Sophie's flat for a stay in Paris! I'd also love to stay there again for my next trip.