Came to Mineral Bar in late September and loved it. It's only about an hour from Sacramento, including that windy road down into the canyon. The river was VERY low, but it was still fun to play around and float away the hot early fall afternoon.
The sites are fairly small and the campground has a funky layout. Squeezing two cars in most sites looked impossible. There is also no water, so be sure to bring enough for yourself.