First, perfect location. I don't know how you could get a better location. It is a big apartment and very affordable. It is a beautiful and very old appartment building. Amazingly quiet for a place that is right in the middle of everything.
Note that there is no elevator and a lot of stiars. Also, if you are there in July/August the lack of air conditioning could be a problem.
Downsides? This is not an upscale BnB. The kitchen and all three bathrooms need updating. The kitchen is small, one-person-at-a-time small. We gave up on trying to cook for the family and went out a lot. It is Paris so going out wasn't a horrible experience :) The appliances are all very old. The washer and dryer were not working when we were there. We aren't sure we would have wanted to use them even if they were working.