This was our third Viking cruise and we've signed on for a fourth in 2018. We fine the balance between water travel and shore visiting much better than ocean cruises. We like the small ships and the congeniality on board at meals and in the evening. While we encountered low river depth on one of our previous cruises, Viking covered it gracefully and we never really noticed the issue.
We did notice on the Danube that there was more tourist river boats so ports were more crowded and we were tied next to other Viking ships from time to time - (but we're boaters and used to do that all the time!).
Food was delicious and the staff accommodated our needs to be a bit isolated in the dining room due to my wife's serious seafood allergy. The wine service was suitable and the evening entertainment was fun - one evening's music contest really got my wife into the dancing!
Small, as expected, but clean, easy to move around in and sufficient room to store our gear - we've learned to keep our bags relative small so we don't exceed the storage - and we've learned to stretch the time between laundry washes.
Did the pre-trip here and found our way around on our own. Fun city but required time to learn the bus system. Los to see on both sides of the river.
Our guide had a friend, also a guide, who was working in Chicago (near us). I do tours in Chicago, and met her - by accident - when she and her children come on my tour! Fun serendipity!