Beautiful flat, would be luxurious for two people and for us (a family of three) it was very comfortable. You could fit 4 if both children sleep together on the sofa bed. This isn't a hotel in disguise: it's a real home, with everything you need to spend a week or more in comfort. The bedroom has a window that opens onto an inner courtyard, so if it's warm you can have some fresh air without hearing traffic.
The bathroom is a bit cramped. Fine for adults one at a time, but children will make a mess - then again, this is central Paris, so what did you expect? :)
The location is very convenient - not only are there several shops within a 5mn walk, there are a few bars/cafes/restaurants too. Metro line 9 is less than 5mn walk away, And Place de la Nation is about 10mn, with many major lines to get into the tourist areas
Finally, Guillaume is a great host, calling us a week before arrival to give us all the details. Their friend and neighbour were there for check-in, the flat was spotless
All in all, a great week in Paris!