The apartment was perfect for our family with two 16-year olds as they had their own space. It's clean, modern, with a spacious top floor. I have a slight hip problem, so negotiating the stairs was tricky and for that reason, I wouldn't recommend people with major mobility issues stay there.
The other potential problem is that the apartment has digital access. Four codes are sent to you 15 minutes before check-in; these provide access to the street door, two gates and the apartment door. Hence the apartment is very secure and as long as the inhabitants of the neighboring apartments are co-operative (ours were), then it's also quiet. That said, I can understand how some people (perhaps especially older people and those who want to escape their appliances for a while), may find this reliance on digital access irritating and unnecessarily convoluted.
Rue Tardieu itself is in the epi-centre of tourism, so souvenir and fast-food shops abound, but on the plus side, the funiculaire is directly opposite the apartment, so you can easily access higher areas, and the view of the Sacre Coeur from the street is beautiful.
Down the hill (south Pigalle) is more interesting and less touristy the further down you go. Rue des Martyrs, after you cross Boulevard Barbes Rochouart, is gorgeous, with a variety of specialty French shops, cafes and restaurants and a cute square with a merry-go-round, trees and a brilliant bookshop selling old and new books from every continent.
My suggestion is to go with at least a little French language under your belt. We all speak some French and always found people very friendly, but we witnessed many tourists beginning interactions with locals in rapid-fire English and without a "bonjour", who were understandably met with annoyance.