Vienna is an amazing city, and Gudrun and Armin's place will not disappoint! It is very clean, the beds and pillows are well kept, and the walls are filled with Gudrun"s lovely art! Be mindful that there are steps to get to the flat, but this did not bother us as we liked the workout! It is also a quiet building with thick walls, so you will have no problems sleeping. It is actually one of the oldest buildings on the block, which was another feature we loved about the home. Moreover, Armin is a very kind host who went out of his way to spend time with us and point out the best local attractions!
Regarding location, it is absolutely perfect! Most tourist sites are within walking distance and the neighborhood is filled with lovely shopping boutiques (our favorite was Trachten-Witzky) and fabulous eateries (You must make a reservation or pop into Salm Brau next to Belvadere castle- they had amazing food and impeccable service!). It is also very close to the Wachau-Valley-Small-Group-Tour-and-Wine-Tasting, which was phanominal and worth spending the day doing! On top of that, it is just a block away from a great gym called "Holmes Place," and a gourmet grocery store named "Merkur Hoher Market," the latter of which will have everything you will ever need including delicious pre-made meals! Just remember that the markets close at 7pm!!
Overall, we had a wonderful experience, and we would highly recommend renting from Armin and Gudrun!