We just returned (a few pounds heavier, and heads still spinning) from a fabulous experience on Uniworld's SS Antoinette from Amsterdam to Basel.
Each and every crew member was pleasant, gracious, and on a mission to show the guests a great time. The food was fabulous! There were sumptuous breakfast and lunch buffets and multi-course plated dinners with freely flowing wine.
The ship is beautifully decorated and immaculately clean. There were a couple first-season technical glitches (TV system took a couple days to settle down), but one could tell that these issues were being actively addressed and didn't detract from the experience in any way. After all, who wants to watch television when around every bend in the river is another photo-op of a story-book castle? There is wifi available, but don't plan on heavy-duty internet access through the satelite system - particularly during heavy use periods. The only other thing of note is that the Rhine river level is lower due to lack of rain and we had to be bussed into two cities (approx 20 minutes) because the Antoinette wasn't able to go as fast in the low waters. No real issues or inconvenience at all.
The cruise offered four optional tours (35-59 Euros PP each). Heidleberg and Marksburg were great and worth the price. Those who attended the optional Rudesheim Dinner reported that they really had a fun time.
The tours guides were universally knowledgeable, kept a good pace, and were pleasant throughout. There are accommodations for those who had trouble keeping up the pace, while allowing them to have an enjoyable experience.
Bring good walking shoes, a high-capacity memory card for pictures/videos, a small notebook to jot down interesting facts (and to correlate pictures with names of castles & churches), and I'd recommend buying the fold-out "Long Rhine Tour" (available at the front desk) to help identify the various locations as you cruise the castle stretch of the Rhine. There's just way too much to remember.