My experience was mixed.
On the positive side, the place is in an excellent location - fairly close to the centre but not in a touristy area. Leopoldstadt is a foody district, safe and pleasant, and contains a great local market, Karmelit.
The building is in good condition (though it has no elevator which can be difficult if one comes with bags).
The heating system works perfectly and the place is nice and toasty even during sub zero winter days.
The apartment is miniscule but it works for a short period. In theory it can fit up to three people (there are three beds) but that gets very crowded. Most of the time, one person or at the very most, a couple can be comfortable.
The hosts are responsive and try to accommodate guests where they can. For example, I was allowed to check in early in the morning and receive guests.
On the minus side, the internet was very unreliable in the flat. The hosts tried to fix this but did not succeed. It was very frustrating for me because I was in Vienna for work and I needed to be connected all the time. If one is just on vacation, it might be OK.
It is difficult to cook in the kitchenette because there is no ventilation and the hot plates heat up the cabinet drawer underneath, which is perhaps unsafe. Not being able to cook much might also be OK if one is willing to eat out most of the time, or buy takeout.
The flat includes most basic furniture but everything is a bit on the cheap side. For example, the bedding is synthetic and there is no shower curtain installed. The laundry is shared and the machines are often busy so one needs to plan the washing ahead.
These issues could be easily resolved with just a bit of effort and minimal expenses on part of the hosts.