We have been on three river cruises and 12 ocean cruises.
Two river cruises were with a company that later sold their cruise ships to Viking and one with Viking.
The 18 day Viking cruise we took last April/May on the Danube from Passau to the Black sea and return was the best cruise we have ever taken. The ship we were on, Viking Primidonna, is normally not used for English speaking passengers. The entire crew did an outstanding job. The Austrian Chef and his staff were so good that the passengers wanted to have a raffle to determine who would be able to take the Chef home with them. We had two small problems on the ship that required us to correspond with Viking after returning from our trip - Viking responded immediately and resolved these minor problems very professionally.
You will enjoy your Rhein River cruise. The stretch of river from Bonn to Heidelberg is amazing - Enjoy looking at all the castles along both sides of the river.
We leave this Friday for a cruise on the Oceania Insignia.
Good luck with your river cruise!