This is a place I would stay at again in Paris, but it is not a place that would suit everyone. The bedroom being in the cellar means that it is extremely quiet and cool, but access via a small spiral staircase means that anyone with mobility issues - or even larger people - would likely have great difficulties, and it is impossible to take larger baggage down with you. In those cases you might have to use the main-level sofa-bed instead, which would take over the living area. The building itself is quite old, so don't expect it to be sleek and spotless.
That said, there is a LOT going for this place. It is relatively well equipped. It is ground-floor level off of a central courtyard. The location, in the 10th arrondissement, is great - a short walk to the metro hub of Place de la République; a shorter walk to the Canal St-Martin. While the apartment is on a quiet back street, it is surrounded by streets with a number of small, very popular restaurants, bakeries, and shops.
Romain was very personable and helpful too; he met us for checkin, and made checkout as simple as possible too.