I stayed at Denys' for a Congress at messe Wien and it couldn't have been better! Extremely close to the venue (literally 5 min!). The apartment is nice, it's student housing so it's very basic but efficient. It was clean and I had everything I needed, definitely recommend it! It's not in the centre but it really close, youll be in Central Vienna in less than 15 minutes (metro line is 2 min walk from the apartment).
Overall, GREAT STAY. I'd book again!