We stayed five nights in Sophie's apartment, and our stay was just perfect.
The apartment is by no means tidy, but it has a comfortable, well-lived in feeling about it: books are everywhere and you can find useful tools: scissors, tape, a cork for your opened bottle, etc. Communication and check in and out were wonderful, Sophie is very present even if she's not there.
The area around Saint Eustache is absolutely magic, with bars and shops of all kinds everywhere. Very lively and close to attractions such as the Louvre and Notre Dame. RER and metro are close, you won't need a taxi most of the times.
We hope to go back soon!