Over all, staying at Ferdinand and Lieschen's place was a very good experience. It is very spacious, well decorated, and has a good vibe to it. It is relatively close to the center of Vienna. Easy access to all the things you will need for your stay (Drugstore, Groceries, restaurants, and cafes). We decided to walk most of the time and it took us about 15-20 minutes to reach the opera house. The penthouse is clean and the hosts are easily accessible for any problems that arise.
I have only 2 real issues about the penthouse. The first deals with the water heater. For unclear reasons, it kept shutting off several times during the day. I did notify Ferdinand the first time it happened and he responded quickly. that enabled me to repeatedly fix the issue during the course of our stay.
The second problem was the air conditioning. I am sure most of the year when it is cooler, it is not an issue. During our stay, Vienna was unseasonably hot with temperatures nearing 90 degrees F or 32.22 degrees Celsius.
The air conditioner is not located in the bedrooms. Fans are available but mainly serve to circulate air. After 24 hours, the apartment was not as humid. During normal temperatures the air conditioning situation should not be an issue.
On a positive note, the hosts were wonderful. They rapidly responded to the issue with the water heater.
If you are travelling to the airport, I would recommend the company listed in their well detailed house manual.