I have stayed in Isabelle's apartment for two months, and could literally not be any happier with it. I agree wholeheartedly with the existing glowing reviews and recommendations, and will just reiterate the main points:
The apartment itself is very nice, small but very well planned, making it feel roomy, and with modern and pleasant design. It has a fully equipped kitchen, from the essentials like stove top and over, to the luxuries like Nespresso machine and a dishwasher. Having a separate bedroom made life much more pleasant, particularly when I had guests over.
The location could not be any better. Rue Mouffetard in particular has very large number of nice restaurants and pubs, many of them catering more to locals and students rather than to tourists, and therefore being much cheaper than the Parisian standard. There are countless bakeries, butcheries, fish mongers, grocers, etc., as well as a few supermarkets within few minutes walk, and that's without mentioning the 3-days-a-week farmer's market. It's less than half an hour walk from most of the Parisian tourist attractions, and 1 minute walk from a metro station, efficiently connecting you to the rest of the city.
And, last but definitely not least, the hosts themselves: Isabelle and her husband were very nice, helpful, and communicative, both before my arrival and during my stay. The few times I had to contact them with an issue, the reply and assistance were prompt.