Will start by saying this was a wonderful experience. Would sail with them again in a heartbeat.
We had read and viewed the wonderful photos posted by a Cruise Critic regular, which you can see at http://www.thepreismans.com/ and I highly recommend that you do so if you are interested in river cruising. As a matter of fact, look there before you go on with my review, it's that good!
We were a group of five, all experienced ocean cruisers. We began with a few days in Vienna on our own (Renaissance Imperial Riding School hotel) then took a train to Budapest, staying at the Marriott.