The Danube cruise with Viking was wonderful--modern ship, comfortable cabin, excellent food, great tours, and--best of all--crew members who were pleasant and efficient.
On some moorings, we had to walk through other cruise lines' ships--that made us really pleased with Viking--our ship was tastefully, cleanly decorated with lots of light; the others were dark and overly furnished (one with gilded mirrors covering the walls)!
Viking's itinerary was interesting, informative, and not overly tiring. We added five days on our own at the beginning of our trip in Vienna--loved it; then we had three days with Viking in Budapest--learned a lot; at the end of our cruise, we had three days with Viking in Prague--fascinating; and then we went on our own to Barcelona--have to return because we couldn't get tickets to enter Sacred Family (had we been with Viking, I'm sure that problem would have been anticipated and solved for us!).
The cabin was well designed--suitcases fit under the bed; bathroom was compact but not claustrophobic; veranda was pleasant.