Very middle of the pack hotel, definitely not worth the €185,- I had to pay for most nights.
First time I encountered a shower that could not be set to the correct temperature, either cooking hot or not warm at all and constantly switching.
Room was next to a busy road that also leads to the hospital so also quite a lot of sirens.
Room is nothing special, not good, not bad, overall state of the hotel is pretty old/dated.
Breakfast also not really special, seen worse but also definitely better. Could use more vegan options and bigger bowls.
Spent 2 weeks in Vienna and had to switch hotels during the weekend. Comparing overall experience and prices I think the Donauwalzer is way too overpriced for what you get compared to the Harmonie hotel I was in the first week.