We had an overall good experience at Josef and Celine's place. Josef's sister kindly welcomed us and showed us around. The appartement is a very peculiar just like you see in the pictures. It's very narrow yet well designed to make everything fit in place. You should be willing to climb narrow stairs to get to bed though, and when you reach the mezzanine, the ceiling is very low so better watch your head!! Not recommended for people with low movility. My wife is pregnant and she could manage but it wasn't the most confortable thing for her. Location is convenient and easily accesible using Vienna's metro and S-Bahn system.
I should point out a couple of things that I think could make this place way better though : for one they don't provide towels, and for a traveler that could certainly be a plus. Towels are big and take a lot of space in your bag, and I can't find enough reason why they decided not to provide guests with them.
Secondly, the architecturally smartly designed appartment could really use more shelves in the bathroom and kitchen to store things. Bathroom is small and having one or two shelves where to place your shampoos, toothpaste, deodorants and such should be an inexpensive yet very useful solution. Same goes for the kitchen.
Other than that, we had an overall good experience so I do recommend this place!