A great experience! I recommend wearing old clothes as it gets a lot of dust in the dunes. But it's worth it. Santa Monica beach, but the whole coast in general, is magnificent. And running around the dunes driving a quad is something beautiful. The team is very nice and friendly; the option to pick you up and drop you off at your hotel is the icing on the cake. I recommend also bringing a bandana, or alternatively buying one from them for €2.50, because otherwise you'll go home with a mouth full of dust. But it would still be worth it for the wonderful landscapes that can be admired. Highly recommended.