Catherine's flat was lovely, and it is surprisingly big for Paris, with many amenities. We were incredibly grateful for the washer and dryer, Nespresso machine, big (queen-size) comfy bed, and the bathtub.
Check-in was easy, and Catherine allowed us to stash our bags in the morning before check-in, which made life much easier for two very jet-lagged travelers trying to find lunch.
The location is excellent too. We liked being close to two métro stations (Alésia on the 4 and Pernety on the 13), from which we could access pretty much anywhere. It's also very walkable to locations on the rîve gauche (south bank of the Seine) - Montparnasse Cemetery, Luxembourg Gardens, Montparnasse Tower, etc. I think it took us about 25 minutes of walking and métro to get to the Louvre.
The location isn't just great for easy access to other sites; the neighborhood is lovely. It's not as busy or loud as some other parts of the city, but it still has many things to do. We liked going to Square du Chanoine-Viollet (5-minute walk) after grabbing breakfast at La Fournée D'Augustine or Le Grênier du Pain. We also enjoyed strolling through the Montparnasse cemetery and getting lunch or dinner at Chez Papa (10ish minute walk). Very close by, there's a great Chinese restaurant (Yum), the best bibimbap I've had (Raon), and an elevated French-Italian bistro with an excellent wine list and delicious sabayon (La Grande Ourse). Finally, if you're here on a Wednesday or Saturday, it's worth walking to the Marché Edgar-Quinet to grab lunch and supplies before heading to the Luxembourg Gardens for an afternoon of hanging out on the grass.
Also, the flat is very close to several grocery stores.
Hopefully, you can tell by my review that I loved my stay.