We had an amazing time at Sebastien's place. It's located in a very central area, full of style and bustling cafes. Sebastien himself was very welcoming when we arrived and showed us around the apartment, which was great. He also went out of his way to pick us up a travel phone card, and I simply reimbursed him when we met (I recommend picking up one of these travel SIM cards if you're from out of country. They are from Orange, and are called Orange Holiday Europe. So awesome to have access to data on your phone to see maps and info without having to pay for expensive international phone plans! 40 Euros for a two-week plan).
Sebastien was also very helpful and available when we had questions during our time there. We simply called or emailed him, and he responded promptly. It was a real comfort for us.
The apartment was far more quiet than I had expected it to be. We rarely every heard another person, and there was virtually no noise from the street, especially in the later hours of the night. The beds were comfortable and we all slept well (aside from jet lag!)
I would highly recommend Sebastien's apartment and would gladly return to stay there again if we return to Paris!