TLDR:
PROS:
a) Super central,
b) tidy,
c) good internet,
d) fast responding and helpful host and
e) a good price (especially for Swiss standards)

CONS:
a) Close to train tracks not for bad sleepers.
b) we had bad luck with the washing machines (often full, people took laundry out of the dryer. This is of course dependent on the other guests) and
c) only one key.

I would book it again.

My wife, child and I spent four weeks in the apartment in the third floor.
The biggest plus is the location. It is close to supermarkets (organic, discount, full-line markets and even a farmers market). It is close to any means of transportation you could wish (tram, train, airport) and it lies in the 110 zone of the public transportation system, which makes it easy to commute. I worked at the ETH and found the way very convenient. The neighbourhood (and possibly most of Zurich in general) is super safe, I would not feel uncomfortable in the middle of the night.
The apartment was tidy and equipped with everything necessary, but not with much more. I liked this minimalistic design mostly, sometimes a little more kitchen utility would have been nice. The dish-washer worked greatly. In our four weeks stay the apartment even got cleaned once.
The biggest minus is also caused by the location, the train-tracks make it noisy all day and night. For us this wasn’t a problem anyway but if you have trouble sleeping you should consider it.
The internet is super-fast, we had issues at one point, which were solved really fast. In general Mike was very responsive, whenever we asked something.

As mentioned we had to wait quite a while to get our turn on the one washing machine available for the Airbnb guests. But it is great that there is one in the building, since the are hardly any coin-operated laundry shops in Zurich.