I was staying at Marten's place during my weekend city break in Vienna. Marten is a nice person and I had no issues with communication. He replies to messages promptly and is very helpful. Furthermore, location of Marten's place is very convenient and all the main attractions of Vienna are only short walk away. However, unfortunately the standard of the place is not great. The flat was very dirty, particularly the kitchen and bathroom. Furthermore, the flat was quite cold.
If you are a student travelling on a budget and you are not fussy about living conditions I recommend this place. However, if you want to stay in at least clean place, I don't think this place if for you.