Just had a 3 night stay in Stefan's apartment and it was really great - so much so that we actually missed it the day after we left Vienna. The apartment is better than even the pictures that are shown in the listing. Stefan has gone to great lengths to make sure everything you can think of is available in the apartment - including Netflix!!. Communication with Stefan was great too - he contacted me ahead of time to arrange for the check-in and everything went smoothly.
The location of the apartment is pretty central for its price range and there is a bus stop right near the apartment and a metro about 5-7 min walk away. Those who don't mind a bit of a stroll can get to the centre of District 1 in 20 minutes or so. We used uber quite a lot - it was pretty cheap (around 4-6 Euros to get to the city centre and back).
All in all - we had a great time in Vienna and Stefan's great apartment played a big part in that.
Highly recommended!!