First, the location. The apartment is fantastically located, the Rue Mouffetard is lively and fun, with everything you could want - fresh fruit and vegetable stores, cafes, ice cream and crepe places, and if you keep following it north you get to Notre Dame. In other words, everything is walkable, and I walked to Centre Pompidou in around 35 minutes.
Second, although small, the apartment was clean and has all the basics, including a cooker, clean bathroom, comfortable bed, and some basic foods in the cupboards. It was perfect for one person, would be fine for a loving couple, but would be a struggle for more people to fit in.
Third, although there was instant coffee in the cupboard and a pan where you could heat water, there was no 'decent' coffee to be had, nor any means to make it. But just down the road is a classic cafe with good food and reasonably-priced coffee where you can sit down and enjoy it, anyway.
Fourth, there was a hiccup in communication with the host Rosa initially. After a long journey to get there, the place wasn't ready, and I had to go off to a conference talk straight away. So not ideal. But when I returned the place was spotless and ready, and she was relaxed about the check-out time. So, on balance, that's fine.
Fifth, if you need some basic groceries there are a couple of stores nearby, just turn left out of the building and continue up Rue Claude Barnard.
Sixth, if you come from CDG airport, don't get to the place by looking up Google Maps. It's misleading. Take the RER to Port-Royal and walk 1.1km - much faster, as long as you don't have heavy luggage.
Seventh, I came to Paris for a conference but did have some time to wander around - I really want to return to this location. If I bring my family though I'd need a larger apartment!