We had a very nice stay at the Karlskirche 3 unit. Thomas met us when we arrived from the airport, showed us around the building and the unit, explained how air conditioning, etc. worked, gave us pointers about transportation options, grocery stores and warned us about the upcoming holiday and grocery store closures. During the stay, Thomas was easy to reach and very helpful. An issue with a shower head was fixed the same day.
The unit matches the pictures very well. It is clean, spacious, modern, stocked with basic necessities, and has plenty of storage. The bathrooms have toilet paper and liquid hand soap - bring your own body soap and shampoo. There are regular mirrors in the bathrooms, I wish there was a full-length mirror somewhere in the unit. Bathrooms have plenty of room to set out the toiletries. There were minimal dry foodstuffs (salt, sugar, and a bit of oil). Dishwashing soap (regular and for the dishwasher) and a new dishwashing sponge were supplied. American travelers should be warned that the upstairs bathroom with the bathtub is European style intended for baths, not showers, but one can definitely manage. Check-out requirements are minimal and easy to meet.
We generally ate breakfast and some dinners at the unit, getting groceries at the nearby supermarket or the market. Fruits and vegetables at the supermarket are of good quality, with better selection earlier in the day. Bring your own bags to the store and market - there are several under the stove in the unit.
The unit is located in a nice building and is secure. The neighborhood is generally quiet - keeping windows open would let in background city sounds but nothing that disturbed our sleep and rest. The building is steps away from Karlskirche (where concerts were held on several nights of our stay), a park, and the U-Bahn station. The Ring Straße, the pedestrian shopping area, and the museums are all within a reasonable stroll. If you want to save the walking for the museums, you can ride public transportation for a couple of stops. The city also has a number of motorized scooter options, and several were usually available at the park.
Overall, this is an excellent unit in all respects, and the hosts are friendly, accessible and helpful. Next time we come back to Vienna, we will definitely consider staying here again.