Great location + incredibly thoughtful setup = super stay.
First off, the negative: it's on the fourth floor without an elevator. For anyone who has read the description, that shouldn't be a surprise and is pretty typical for buildings that are both small and old. That is really about it for negative points.
For positive points, where to start. The location is as good as it gets. Right at one end of rue de Buci, square in the heart of St-Germaine-des-Près, the 6th arrondissement. Restaurants and shopping for every taste.
The location is perfect whether a tourist or, like is us, that someone who has lived in Paris for some time (including in the 6th). Less than a five minute walk from the Metro 4 Line, which is ideal for those arriving by train or airplane (airport transfers at Châtelet for a quick 10-minute ride to chez Vincent).
As for the apartment itself, it is simply the best use of space and design that I've seen in my many Airbnb stays. Despite an overall size that could be considered small-ish, the feel is anything but, mostly because of Vincent's very clever use of space-saving design. Unlike many space-saving efforts that I have seen, Vincent's is smart, stylish, and done with a spare-no-expense effort that reflects the fact that Vincent hired professionals to do everything. Case in point, the use of an HDTV projector in lieu of a TV was perfectly designed by incorporating a very well thought-out sliding drawer design. The apartment is full of similar examples, from the discrete but accessible placement of the water heater to fitting in a very comfortable queen-sized bed. Our family was five people (parents, two young children and a baby) and we never felt cramped.
Finally, about Vincent - what to say? He was unquestionably the best host that I have encountered on Airbnb (and there have been some exceptional ones). Responsiveness and unavailability were near immediate and very graciously given. He emphasized a desire to assist and it was not hyperbole. To help with my children's food sensitivities, we needed a larger-sized freezer than was available. Not only did Vincent offer to store our frozen foods off-site, but when we declined, opted to install a freezer in the apartment (and yet another clever installation at that).
Throughout our stay, Vincent remained in touch, ensuring that we did not require any assistance and offering it if we did all while managing to not be invasive or probing. He was, quite simply, very helpful.