We have recently stayed for 3 night in Apartment Karlsplatz. We have greatly enjoyed our stay!

the apartment is clean and well furnished. and it is located just across the road from one of the main U-stations - U1 -(literately across the road). This was extremely convenient for us. it is also located close to the Center (Karlsplatz and Nash-market are within 10 minutes walk, so is the Belveder). To get to Stephanplatz, the Opera and all it would take using the U, like 10 minutes in total. Also this is the same U to get to Shonbrunn and to the Parter area ( this is also the area where night clubs are located).
So the Location was excellent. It is situated on a quit street so no noise was heard and we could easily sleep in late.
the apartment is equipped with all necessary stuff for long or short stay. we really enjoyed our staying.