This apartment is an absolute gem and I've stayed in many Parisian AirBnBs in the past.
It's is literally located across from the Gare Montparnasse next door to a large supermarket. Even though the entrance in on the main Avenue. Once you enter the main gate, the building is all the way to the back and the 5th floor apartment faces a back court so there is absolutely no street noise even though you are right in the middle of the busy Montparnasse area.
The apartment is very stylish (with lots of cool African art) and modern. There is a large bright living room with comfortable sitting area, a private bedroom and 2 bathrooms and the 2nd (foldable) bed (interestingly located in the large kitchen) is SO comfortable and large that my wife and I decided to let our 15 year old daughter sleep in the bedroom and preferred the foldable bed instead! Bedding was top notch.
We did not get to use the kitchen but it looked fully featured. Note that there are a full set of stairs to come up to the apartment from the main entrance so you'll need to take that into consideration (entrance on the 5th floor with elevator but apartment technically on the 6th floor).
It was January so we did not get to sit on the cool balcony (there is also a second one on the other side of the apartment) but I can imagine that grabbing a coffee there in a warmer season would be very enjoyable!
Quentin, the host, was very fast to respond, super nice and extremely accommodating for the checkin/checkout... This is probably the best AirBnB I stayed in Paris in the many years I've been coming. Highly recommended. 6 stars!