I stayed at Margaretha's apartment for nearly 4 weeks on a work-related visit. I would definitely choose it again if I were to return.
The room is spacious and airy, and although it opens onto a relatively busy street, the sound proofing is good. Nice views towards the Liechtenstein Palace. The room and bathroom were very clean and recently renovated - the overhead shower was also a nice bonus. The wifi was fast and flawless throughout my stay, and there is also plenty of room to work on laptop if necessary (sofa, kitchenette table, and chairs).
The kitchenette has a refrigerator and basic amenities (pots, pans) to make basic meals if needed, and also a capsule coffee machine. There is no oven or microwave. There is also a relatively large TV, which I had problems operating (couldn't get the channels to show), but left it at that because I didn't really intend to watch much anyway.
I found the location of the apartment perfect, with excellent cafes and restaurants nearby (try Riva for proper wood fired Neapolitan pizza), and also a relatively short walk to e.g. the Museumsquartier or the old center. Public transportation is also readily available. The canal and its waterside eateries are also nearby (open during the summer).
Margaretha was a welcoming and warm host and always ready to help with any questions or problems.
Finally, although the sound proofing is good towards the street, it is less so inside the apartment. Remember, you are literally staying next door to Margaretha's home, and she obviously has a life of her own, coming and going, maybe having people over, and so on. So if you are a light sleeper, having ear plugs is a good option. Despite the shared entrance and next-door living, I felt the apartment to be private enough.
In sum, I would return to Margaretha's place without hesitation and recommend it highly to anyone planning a stay in Vienna. Excellent price/quality ratio.