The apartment has a perfect location - just between the Opera House and the railway station. We're used to walking a lot so we used public transport only once, all the other time we were exploring the city by walking. There is also Belvedere and Naschmarkt (market) not far.
Even though we didn't meet our hosts, everything was really nice and we didn't feel any discomfort. There is also a shop just at the entrance which was also very convenient.
If I ever travel to Vienna again, I'd love to stay in the same apartment again - so I guess there is nothing else to add :)