Location
The location is perfect for Montmartre. Rue Caulaincourt has lots of bakeries and bars and supermarkets within a ten minute walk, and there's a small supermarket down the stairs outside the apartment block which is open from 8:30-9pm most days. It's less than ten minutes walk to Lamarck-Caulaincourt station. The apartment was soundproofed enough to not be disturbed by anything outside, and the area felt safe, though with young children we weren't out later than 9pm.
Apartment
The apartment has a large living room with internet TV, the wifi is fast and reliable, and the apartment was a good temperature for us in February. The kitchen had a microwave, bean-to-cup coffee machine, fridge-freezer, oven and all utensils we needed.
The beds were comfy, the showers worked well. The apartment is very nicely decorated and it was fun to live like a local in such a nice apartment for a few days.
Communication
The agent who managed the booking was mainly communicative, responding quickly until we had one problem which I'll list here in case it happens again: the key for the apartment front door stopped working on the second evening and the agents had to send someone round to free the lock while we waited on the landing outside the apartment. No one else has mentioned this though so I assume we were just unlucky and it isn't a regular problem. After the agent freed the lock, it continued working for the rest of our holiday.
Overall, it's a lovely, well equipped apartment in a great location.