I really enjoyed staying at Karl's place. My only regret is that I was so jetlagged when I arrived that I didn't talk with Karl longer; I enjoyed very much the time that we did chat. He was responsive and thoughtful, even providing a few items for breakfast so that I wouldn't have to run to the store as soon as I arrived. The apartment is beautiful and sunny, and is a nice size for 1-2 people. It feels very private and is very comfortable with all of the amenities that you would expect from a well-appointed home. All of the appliances in the kitchen are new, and they include a full stove and oven, a dishwasher, a toaster oven, and a toaster, and a good-sized refrigerator/freezer. The bed linens were crisp and comfortable, and the towels fluffy. I particularly liked that everything smelled so nice - warm and inviting fragrances are found in the bathroom and toilet room. They were not strong, and could be avoided if not your thing.
Wifi was fast and reliable, and portable(!) enabling wifi access throughout the city. Karl supplies a nice selection of brochures and menus to help make choices of how to spend your time in Vienna. The apartment is on a well-serviced train and bus line and is directly in front of a taxi stand. While the apartment is not in the center of town, it is in a nice and quiet residential neighborhood, and downtown is just a short train ride away. The Millenium tower is a shopping mall with restaurants and a food court, and is only one block away.
For anyone who may need to avoid stairs, this building does have an elevator, though there are about 2 steps up to the front door of the building and another two just inside the building door. The elevator is small, though the door and space is large enough for a stroller or wheelchair if needed.
I highly recommend Karl's place. I'm looking forward to my next stay in Vienna, and will have my fingers crossed that Karl's home is available again during that stay.