I've been to the Velazquez and enjoyed it a lot, starting from a basis of almost-zero knowledge about the artist. (I like to learn about people new to me!) The audio guide was well worth it and gave a good pace to the exhibition (which took about 1.5 hours). If you're there, I also highly recommend the free temporary display (Manet to Picasso) in the basement of the Sainsbury Wing (and the film clips in the little auditorium down there are extremely good both in themselves and as a way to take the weight off your feet!)