I booked a last-minute break for my family of four, and the owner was very helpful & informative.
The apartment was clean and well equipped - beach chairs & towels were a welcome extra. Although there seemed to be a lot of people staying at the complex, it was never crowded at the pool or noisy at night. The only downside to the apartment was that the second bedroom did not have an air-conditioning unit, only a ceiling fan which was not very effective in the August heat at keeping the room cool enough for the children to get to sleep. Our bedroom was lovely and cool, so that was fine!
Mar de Cristal is small and english is not widely spoken, so some knowledge of spanish would be useful. The local beach is lovely - quiet even in mid August with warm, shallow water. It is about a 10 minute walk from the apartment, but because we were carrying chairs, towels etc, we drove as it was easy to park very close to the beach.
Arena Cafe on the beach is ideal for lunch, ice-cream and drinks. Piccolo Mar, next door is a good italian restaurant for an evening meal. For a better choice of places to eat, you do really have to go to Los Bolones. Although it is possible to walk there, we didn't fancy doing so in the summer heat with children - particularly not the return journey after dark. We were glad we hired a car, and would strongly recommend doing so.
In summary, a lovely relaxing break which suited us perfectly because we wanted to do nothing but read, swim and eat in the sun!