I used the St Nazaire - Gijon ferry in June last year when I was heading down to ride the Pyrenees.
The ferry cost £120 for two people, two bikes and a cabin, it would have cost us more in fuel to ride down - and being a night crossing it's a "free night" of accomodation. Like spending a night in a hotel and waking up hundreds of miles further along!
The ferry was obviously designed for truckers - it's not 5 decks of shops, bars, restaurants and cinemas like the St Malo ferry or the Santander ones. However, the cabins were en-suite and in decent condition, the bar and restaurant were perfectly good and there was a small shop for any little bits and pieces you need.
I'd definitely use it again - much cheaper than the Santander ferry but saves a whole load of the route.