Nicolas' flat was perfect for the first half of our stay in Paris. Its location allowed us to wander in some of the most picturesque parts of Le Marais without ever having to use the Metro.
The apartment is very modern, and we really appreciated having a washing machine and updated shower. The fifth floor walkup might be a challenge for those with mobility issues (or for those who buy too many souvenirs such that their suitcase grows to monstrous proportions). However, if you are able to manage it, you are rewarded with beautiful views.
Finally, checking in and out was easy. I would recommend that at least one person spring for an international data plan (which we, being from the US, needed) so that you can get in touch with the management company as you near the flat.
Again, this is a great place, especially if you are keen on exploring Le Marais!