The flat is accessed via a key that you obtain from a keysafe which is 10 minutes away from the flat. You also drop the key back there when you leave. I'm not even going to mention the stairs because so many other people have but I will say they weren't a problem for our group - we were two adults (40ish) and two 18 year old girls - great exercise!
The key is a bit fiddly in the lock but the apartment itself didn't disappoint - just like the photos. And not dated as someone else suggested! The girls slept on the sofa bed and we had the room and we managed fine for 3 nights (might be harder if longer because not a lot of privacy). Also no problem with hot water (although I was worried with 4 adults) and heating was fine although we didn't really need it because the weather was mild during our stay.
The flat is ideally situated by the Arc de Triomphe and Parc Monceau and the Eiffel Tower is within walking distance. The metro is very easy to use though and we were able to access Montmarte, Place de la Bastille, La Marmais and the Louvre with ease.
Supermarkets, patisseries, bars and cafes are all within walking distance too! So everything you need to make your stay comfortable and enjoyable.
I would definitely recommend and would go back!