Helene's place has an amazing location. The pictures are 100% accurate - you're overlooking the back of the Notre Dame cathedral. From there, you're situated in the heart of Paris, walking distance from tons of great neighborhoods and attractions.
The downside is that the apartment is on the 7th floor of an older building. This means a trek up flights of stairs or via an old elevator and a hot room with no air conditioning during warm Paris summers. The apartment is small but well-appointed and is comfortable enough for two people. It's an older space, which means that though it wasn't dirty per se, it has accumulated some wear and tear over the years. For example, there were some issues with the shower water pressure during our stay, which we were told are being worked on for future guests.
Overall, Helene (and her mother, whom we met during our stay) are wonderful hosts. Any issues we had with the place were minor and not enough to take away from a great visit in a truly special location.