We booked a 14 day cruise with Scenic from Budapest to Amsterdam, and had a very mixed experience. Due to low water levels (not Scenic's fault), we ended up on three different ships (the Amber, the Jade and the Ruby; the original itinerary was supposed to be Pearl). Overall, the facilities, food and drinks were fine and the crews on both the Amber and the Ruby were outstanding. Where things came unraveled were in communications and the transfers between ships necessitated by the low water. We were transferred on the eighth day, by bus, from Passau to the Scenic Jade in Nuremberg, although we had been told that the water was only low between Passau and Regensberg, thereby bypassing 100 km of river. The crew on the Jade were obviously upset about something (some were downright surly for a while). Then on day 11, we took a smaller third party day transit boat (obviously arranged at the last minute and very poorly appointed) from Rudesheim to Marksburg Castle, then we were bused back to the ship in Rudesheim and next day we were bused for over 3 hours from Rudesheim all the way to Cologne, basically bypassing the entire Rhine gorge, even though we saw, from the bus, the Scenic Pearl sailing upriver at Lorelei. After the bus ride, we had to wait in Cologne for another four hours for our luggage; the courier driver had gotten lost, we were told. The tour of the cathedral in Cologne was an unmitigated disaster. Clearly, Scenic had dropped the ball in dealing with the low water level contingency plans. We felt we were misled on several occasions. We were offered C$526 total compensation out of about C$20K paid and haven't heard back from Scenic on our requests for further compensation. We won't be sailing with Scenic again.
Apart from the cathedral, which is extremely crowded, there is little of historic importance in this city as it was basically leveled during the war
Take a good canal cruise, it's the best way to see this very interesting city, and less chance of being run over by the mad cyclists!