We were recommended Viking and this cruise in particular by friends who had just returned from it. It was absolutely everything we had hoped for -- the boat was more than adequate to the task, the staff was stellar, the excursions were mostly excellent, the food was great, and the new friends we met on the cruise were great too. The cruise was extremely well planned, and we wanted for nothing on the trip. The highlights of our cruise were Cologne, Heidelberg, and a wonderful culinary tour of Strasbourg ("Taste of Alsace"). Probably the only disappointment was our last excursion to the Black Forest (a long bus ride with little payoff), but it was one of the free ones.
We lost 6-7 hours of daylight on our pre-cruise visit to Amsterdam due to equipment failure on the transatlantic flight. This, of course, was not the fault of Viking, and they were excellent in helping change things, and met us at the airport without a hitch. Amsterdam was great, the hotel (Amsterdam Hilton) was perfect, and what time we had there we structured ourselves to great success.
Although the boat lacked much standard cruise appointments such as a health club or pool, we found that its size (200 adult-only passengers) was perfect, and those things were not at all missed. We chose the least expensive room, which we recommend, since we only slept there -- too much to do, and great fellow travelers to do it with.
We would strongly recommend our boat (the Kara), and this tour, as well as Viking.
Small and efficient. The window gave us a view of the river, and we were well away from any activities.