We visited the campground at Holiday Harbor for 3 years straigt from 2006 - 2008 and loved every visit. It is true the reservoir level drops substantially from the beginning of the season to the end of the season - it is, after all, a reservoir. We stayed in spaces 4, 6, & 7. One year we thought we would like to back up to the water and chose space #23, but we ended up moving back over to space 7 after trying to squeeze in a 29' trailer with slide - not a good fit. The campground has since changed their policy and no longer allows slide-outs in spaces 23-25, which is a good thing.
The park is full hook-ups, some shade if you pick the right space, and requires a reservation months in advance if you want a shaded spot. Shade is good because it gets very hot in the peak summer months at Shasta. There is a great swimming hole you can access between spaces 14 & 15. Space 14 is an excellent spot for shade and roominess, but you have to contend with the foot traffic accessing the lake path. Once you get to the swim spot, there is a floating dock you can swim out to and sun on or use as your diving board. Although the water level is low and requires you to take quite the hike to reach it by the end of summer, it is also much warmer than early on in the season. So it's a trade-off. You either get a full reservoir with really cold water or a warm reservoir with really low water. I'll take this place either way!
You can use the dock out to the boat slips for a little fishing spot in the evenings - nice.
We rented a patio boat for a couple days and took day cruises to enjoy more of the lake. You can also get to the Shasta Caverns very quickly (it's on the same road as Holiday Harbor). And if you want to take a short jaunt north, you can visit Macarthur Burney Falls (spectacular!) or the upper, middle, & lower McCloud Falls. You may also want to head to Redding and visit Turtle Bay Museum, the sun dial bridge, and the arbortorium. Or you can always take a quick trip to the south end of the reservoir and tour the amazing Shasta Damn - that is a great tour.
Bottom line - Holiday Harbor campground is an excellent home base for those of you interested in sight-seeing the area while being close to water and maybe using the toys available at Holiday Harbor to have a little fun on the water (campers get 15% off the 'toy box').
This review is the subjective opinion of a TripAdvisor member and not of TripAdvisor LLC.