Incredible location. You are right next to Notre Dame. You cannot get more centrally located in Paris. I actually own an apartment in Paris in the 16th arrondissement (little old lady and poodle poop land), which we could not stay at this trip because my sister was using it. But I have never experienced Paris like it is when you stay right there in the middle of it all.
Amazing views. You look out North across the Seine to the Right Bank and the beautiful Hôtel de Ville, and East to the corner of Île St. Louis. Wow! That is the embodiment of "the romance of Paris."
Pierre is a gracious host, helped us when we arrived with our bags, and made suggestions for restaurants and boulangeries.
You should be aware though that staying here feels a bit more like you are crashing in your friend's apartment after your wife kicks you out. Pierre actually lives there, although don't worry—he does move out when he's renting it. So it definitely has a lived in feel.
And "romantic" in such a location, also means a very old building, no elevator, and six flights of stairs up a narrow spiral staircase. So be prepared to get your exercise when you go out for croissants in the morning, and especially when you bring them back. And don't bring too much luggage! (Always a good rule anyway.) It also means an old creaky floor (you cannot quietly sneak around in the middle of the night), and the toilet is in a completely different part of the apartment than the bathroom (remember that part about the creaky floors in the middle of the night?)
On the other hand, did I mention the location and the views?!