We had been with Viking before and generally appreciated their level of organisation and the standards they set on their vessels. We had two nights in Lisbon after flying out from Machester and then embarked the Viking Osfrid at Vila Nova de Gaia on the left bank of the Douro, opposite Porta. Very friendly crew and very high standards of cleanliness and cuisine. Three Viking buses (massive legroom) met us at every port for the day's included excursion led by one of the three highly competent Programme Directors. All days worked well and each excursion was well thought out and nicely balanced, especially the trips to the wine producing "Quintas"..
Downsides are the tiny cabins (riverboats on the Douro are not close to the size of the boats that operate on the Rhine and Danube. Nevertheless they are ergonomically designe and everything packs away amazingly well. Just nowhere to sit which means you rely heavily on the public areas, which are numerous and comfortable.
Dining was by a single sitting in the restaurant and the food was generally imaginatively prepared and tasty with plenty of choice. There is a comfortable bar/lounge which is also used for port/excursion briefings and the information on each following day's activities was accurate and detailed. Also used for evening entertainment when local musicians etc would occasionally perform.
All in all a thoroughly satisfying cruise experience run by a very professional team.