We have cruised many times and our first River cruise with Viking in France was very disappointing, but after a Viking Ocean Cruise which was excellent, we thought we would give Viking River Cruses another chance and we're pleased we did.
The Duoro river is small so not busy, but the scenery was the real surprise, really beautiful, steep cliffs stepped with vines and olive trees which all have to be harvested by hand. The Locks are a feat of engineering, and it's really interesting to sit and watch as the Helgrim navigates through them. Don't expect to dock up by small towns in the evenings....there aren't any.
We only went on the included tours, the Mateus Palace and day trip to Salamanca our favourites.
We were on Deck 1 and although the cabin was 'cosy' there was everything we required. We had problems with the temperature of the water in the shower and that was fixed quickly. If you have trouble sleeping perhaps deck 1 is not for you as you can hear the flushing of the toilets on the higher decks, this would go on quite late and start again early in the morning, not ideal for a good nights sleep.