My best friend and I took the Rhine River Christmas Market trip. All-in-all, we had a good time but we probably would not do another cruise.
Pros:
The boat was in good shape and clean and well maintained
It works well to travel mostly at night and have the daytime to explore
Most days, everything was well organized
Our fellow travelers were very nice
The guides for the included tours were funny and knowledgeable
Cons
THE FOOD I was terribly disappointed in the food. As someone else stated, "The display is better than the food". Food quality is a big thing for me.
Expensive
The Christmas markets were so crowded you couldn't move at some of them (not Viking's fault}
The Christmas markets had very little unique or handmake items
I felt ripped off by the taxi the staff ordered for us at the end. The taxi to the airport hotel was 80 euros but a Uber back to town was only 22 euros
The staff were nice but at times not very helpful due to lack of local knowledge. Some of the next day briefings were too long and unnecessarily convoluted.
The only outing we paid for was disappointing, I think because it was winter and the Fort was partially closed, it lasted 1/2 as long as it was supposed to.