We enjoyed our stay in this one bedroom flat. It was very clean and Ralf made sure there was some complementary food for us which was very nice. Ralf was also very helpful and easy to reach as he gave us very detailed directions to the place the on the day we arrived in Vienna.
The flat itself is 10min walk to Belvedere Palace and about 13min walk to Rennweg station from where there are direct transport links to city and airport.
The only issue which we had was with the wifi as it was appearing and disappearing and it continued to have issues even if we re-booted the router. But luckily we could use our mobile internet instead so it was not such a big problem for us.
Overall, we enjoyed our stay in this flat and I would recommend this place.