We did this same stretch of the river Danube last summer with Viking's Grand European Itinerary on Viking Odin and loved it. The December Danube Waltz itinerary might sail down the same river but is a totally different experience.
I don't think I truly appreciated what we had seen sailing from Passau to Budapest, stopping at Christmas markets along the way until returning home. Then, it seemed as though we had been to a magical land that was a unique experience. Sizzling bratwurst on the grills of those markets along with Ghluwein, a potent holiday beverage were made possible with a tour included in the price at every port.
On the ship, with less than 200 passengers, the crew is happy, attentive and most know you by name. I have to say, I can't remember the last time I felt more like tipping at the end of it. They really worked hard to take care of us every step of the way. I liked that too; from the time we collected our luggage upon landing in Germany until we were on our flight coming back home, Viking was with us. Actually, they started prepping us for the trip in advance of sailing with a doc package (they still send them) that included nice books on the places we would see, a day by day detailed itinerary and more.
Living in Florida, we are not big fans of cold weather and we sure got plenty of it on this cruise. But it was totally worth being a little cold to see the amazing things that come around every bend of the river. I'd do this again in a heartbeat.