It's incredibly beautiful. I went there when it was around 3pm in the afternoon. It was rather cool as well during that time of the year. The water is so cool and refreshing.
What is mad is the seagulls trying to find food everywhere at the beach, on the beach, near the beach, over the beach, and bird droppings. They are super fun creatures though, and incredibly intelligent.
I would say even with the seagulls around, the beach is rather clean.
It's rather quiet for a crowded beach. There is a lot of beach activities going on - swimming, volley, football etc
Numerous restaurants around the beach area, so you can enjoy foods around if you plan for a whole day out at the beach.
It is so near to town centre (I gotta give a clap to the people who planned the city of Barcelona. Everything is walking / cycling distance, if you have the time and leisure for it).
