Anonymous wrote:If the river level is low it turns into a bus tour. Which is a poor, but still very expensive, replacement.
This is a good point-- if the river is too low or too high the boats can't go and you will likely be bused places (although different lines handle this somewhat differently).
We did a Rhine cruise a couple years ago and enjoyed it-- the Rhine has some scenic sections and locks which add to the interest.
Don't assume Viking is the best brand because it's the most familiar. AMA, Uniworld and Avalon are all good (if not better) options. There is a river cruising forum on cruise critic which is a good resource if you have specific questions.