The apartment was perfect for our stay and we would recommend it to our friends and family.
The best points were:
1) Location was fantastic because Karlsplatz is in easy walking distance of the major sights of central Vienna. The U-Bahn station is only a few metres away and provided connections to the airport and other destinations, such Schonnbrunn and Prater.
2) The bight and spacious apartment itself. It appears to have been recently refurbished to a high standard. We had all the mod cons as well as a nespresso machine, complimentary wine, food, bottled water and toiletries, and sound system. There are three balconies, including a roof terrace, which provided a nice cool breeze through the flat. There was also very good air conditioning and ceiling fans. The appartment is in a well maintained period building with a functioning lift.
3) Our host was very helpful ahead of and during our visit. He greeted us when we arrived and showed around the flat.