We had a great stay at Marion and Koray's place. The apartment was quite charming, calm and spacious. M & K are fantastic hosts - you can tell they're making every effort for you to have a great time during your stay in Vienna.
I would say the apartment is ideally located : close to the historical center, in a nice and lively area with daily open markets and such. There is a small park just in front of the building, and lots of option for transportation nearby.
To sum it up, I would definitely recommend staying at Marion and Koray's place !