We went in July, and although there were a lot of people, the beach was pleasant and relaxing. It wasn't at all crowded. The water was cold (we're from Miami) so we just enjoyed walking along La Concha. The other beach was much smaller with a younger, surf crowd. La Concha had several couples an families.
Although we visited SB in the height of their season and the beach was rather busier than we would ideally like, because the beach is so expansive we were always able to find a decent spot and we found the water clean for swimming.
We enjoyed our walk around (or along) the beach from our hotel to the old town. Was a really lovely route with spectacular views, and felt safe even very late at night.
The beach was clean, well-equipped and the views of the bay were lovely. A real 'old fashioned' seaside resort - think Brighton or Scarborough with better beaches!
We visited at the end of November so we looked at the beach rather than lounged on it. The views from Monte Igeldo and Urquall are worth the (steep!) climbs.
Two great beaches, the nearest one had great waves, the other was much calmer and had rafts you could jump off great fun also lovely food.
There is a quiet beach for swimming and a beach with more waves for surfing. Both are beautiful and well kept.
Nice large beach, dramatic mountains, gorgeous views, crystal clear water.
Small beach, well equipped with beach chairs, small bars, toilets, etc
Playa de la Concha is one of the most beautiful beach in the world.