We used the Portsmouth Santander servcie in April and travelled on the flagship Pont Aven.
Sorry, not impressed. Firstly, the cabin we were given was a half size one at the very front of the boat, even though I had booked the previous Autumn. It was not a budget one but we could not believe we were expected to sleep in basically a wardrobe. Of course no alternative accomodation was available. My wife was seasick the whole trip, and even I, although have sailed a lot, did have to cuddle the loo a couple of times. Mainly because we were so far forward the cabin went up and down like a seasaw all the trip. This meant we did not spend any money on food, as we were mostly losing it, but from what we did see, we did not miss much. The food in the main restaurant was mostly chips with everything and the fish was so old looking that it was doubtful when it had been caught. After looking at it, we hurried back to the loo again. There was a swimming pool and jacuzzi on the boat, which we had looked forward to using, but it was closed. The staff were not very impressive either as they were pretty surly and obviously did not give a toss about their passengers. They were French of course, but that should not have made a difference.
I have to compare it with our experience of DFDS where the staff are Phillpino so (I am sure because of their job fears) are always smiling and cannot do enough for you. The food involves 3 different types of food in an nlimited buffet with food from Scandinavian Smorgasbrod through a full Thai menu, plus of course a UK one. The boat seemed more stable, but maybe that was the North Sea and of course the crossing is shorter. The downside was the wine prices were high, but beer was ok.
So, our experience was not good, but others on the same boat seemed happy enough.
The good thing was that when we docked, we wre right in Spoain, but we will not be repating the trip and will stick to the hsorter North Sea crossings.