pizzalady:
Obviously, you have so much input here from TOTVers that you'll never get around to actually doing a river cruise, but your research will make you the ultimate expert!
As a Tauck fan (seven trips with them), I urge special attention to what they offer. Their bottom line often (that is, usually) seems way expensive, but if you delve deeper, you'll find that virtually everything is included in their price.
After the first trip I took with them, I spent lots of time estimating what the trip might have cost me had I made individual arrangements, as opposed to what I spent letting them do it all.
On my own, for a week: about $375 a night.
With Tauck, same week: about $275 a night. $100 less.
SWR