My boyfriend and I stayed at Jarek's apartment for two weeks while we were looking for a permanent place to stay (we were staying in France for 8 months).
The apartment was so much more charming than the pictures! Jarek's place was comfortable for two (and cozy if you have three guests). My favorite part of the apartment is definitely the balcony! I really enjoyed sitting out on the balcony with a glass of wine and people-watching. The bathroom and kitchen are tiny but that's expected for most Paris apartments.
Yes, the building is old, but I thought it added to the charm of the apartment. The apartment is on the 4th floor -- with no elevator -- so be prepared to walk up four flights of stairs. The building is also on a hill (Montmartre is very hilly!). It was definitely a workout, but worth it.
The apartment is only a 5-min walk to Sacre Couer. Even though it's so close to a big tourist destination, the street is very quiet and not at all touristy. The apartment is also very accessible to the metro, supermarket, bakery, restaurants, and bars.
Jarek was a great host. He provided suggestions of great restaurants nearby, and offered some advice for apartment-hunting! He responded quickly to all my messages.