OK, let me start with the topic that seems to be at the top of everyone else's list. Yes, we did have to change ships mid-cruise. We threw our stuff into the suitcases, went on the daily tour, and got back to an identical ship after the tour. The suitcases were in our room and ten minutes of unpacking later we were headed to the lounge with friends. Folks, Viking (or Avalon, or Taulk, or Uniworld) can't control the water level in the Danube. Based on the circumstances they encountered I thought George (our cruise director) did a magnificent job of making the transition as effortless and unobtrusive as possible.
The ships (TOR and VAR) were wonderful for what they were intended to do. By that, I mean they're not ocean going cruise ships with casinos and movie theatres meant to help pass hour after hour of open ocean sailing. They were both immaculately maintained and the staff was friendly, helpful, and above all, available. The food selection was wonderful with a number of local dishes offered every evening together with good old American fare (prime rib, etc) for those less adventurous, and the service was excellent.
Where Viking shines is the quality and variety of the shore excursions offered. The local guides they provide are amazing both in the depth of their knowledge and their ability to engage you in conversation to the point where you know they actually enjoy what they're doing and not just parroting a canned speel from a tour brochure. Over and above the standard tours, the wide variety of "elective" tours available pretty much guaranteed everyone the opportunity to pursue their particular interests.
Excellent - comfortable, immaculate, and well appointed. Not overly large, but more than adequate considering we spent very little time there.