The apartment was great (for its price).
Location was quite beautiful, 35 minutes walking from Belvedere, 40 minutes from Karlskirche and what's the best 20 minutes walking from Vienna Central Station (easy way to airport), 5 minutes to supermarket (Lidl).
The apartment was really clean, except for a small amount of mold near new (!) and comfortable bathtub, but it wasn't a big problem at all. Also, the mattress was too soft (I'd prefer to sleep on the sofa, but for 3-5 days it's okay) and the walls were quite thin (sometimes we could hear what people speak in other apartments and stairways, but the voices weren't loud). But because we didn't spend too much time inside the apartment it didn't bother us at all. The kitchen was nice, it had everything that was needed.
I should mention that the owner was very nice and answered all questions very quickly.