We spent one night in one of the walk-in sites here. We were able to find an open site at 10 am with no reservation, although the drive-up sites looked all full. Despite a stormy night, we enjoyed this campground, with a beautiful view of the lake. It looks like sites in the lower drive-up loop are the most desirable, being right next the the lake edge. The campground is also quite close to Mt. Rushmore, which could be a pro or con.
The Cedar Pass campground was a great pitstop for a cross-country road trip. The sites and facilities were clean and comfortable. The picnic tables had appreciated shade coverings, and the grass was perfect for tents. There was even an informative, minimally hokey Forest Ranger presentation at the amphitheater that night. My only critique is that the campground is not very secluded, and without any tree cover, the sites are not very private. It did make for a great view of the sunset, though.