We stayed here for 3 nights in late September, which is to say, just as the weather transitioned from absurdly hot into climbing-friendly. This meant the best sites were all taken by mid-afternoon and the campground was completely full. Basically, given the proximity to LA, if the weather is nice you need to get here early.
There seemed to be a wide variety of campsite size and quality. I'd recommend figuring out what portion of the day you're likely to be in your site and choosing one based on shade availability at that time. The campsites and generally fairly sandy, with big rocks all around. The Joshua trees themselves are ubiquitous.
The main reason to stay here is the location. In addition to the bouldering problems located right in the camp, there are numerous climbing opportunities within a couple of miles. Hidden Valley itself is just a few minutes away by car, and there are other rocks with name routes within walking distance of the camp.
(Our first night there, a rare storm system was coming through and we kept climbing up the nearby rocks to see one of the best sunsets any of us had ever witnessed. The clouds lingered all night and we were treated to an awesome sunrise as well. I don't think this is a common occurrence, but it was absolutely awesome.)
This review is the subjective opinion of a TripAdvisor member and not of TripAdvisor LLC