We stayed at his place for 6 days - group of 10 ppl. Overall a good experience with few caveats.
The biggest PROS for this place is its location - 8 minutes walk to Louvre museum and 10 minutes to Notre Dame. Gare du Nord train station is within 8 minutes taxi ride also.
The biggest CONS for this place is also it's location - at intersection of very busy streets (Sebastopol and Rue de Rivoli) and traffic light. Lots of people walking by till late at night and considerable amount of noise in the apt (4th floor). Elevator is tight (3 ppl at most) but starts on the 1st floor - you have to climb 15 or so stairs get to it.
Apt is old but charming - we were able to sleep ok - no A/C in the apt, well most of Europe don't have A/C. It was unusually hot in early August (mid 90s) and we had to sleep with windows open - hence a lot of noise at night.
The apt was well furnished but lacked some basic amenities - no extra TP/ no paper towels/trash bags etc. had to buy our own. The cleaning lady who was cleaning when we arrived didn't do a good job - lots of dust left on the furniture and rugs not vacuumed.
Overall, a great place for young people – not so great for families with young children or elderly.