I used to work at knotts soak city palm springs.. also buena park soak city, toward the end of the season and both parks on the weekends had a pretty good size crowd.. At both parks in june through august both parks are packed ... and there are usualy soem decent size lines ! But good fun any way ! working there was a good experince !
I have the Knott's Premium Pass, which includes the Soak City Parks. I don't know if it is still available, but we bought them at COSTCO for $75 last year.
Earlier in the summer, Buena Park had a good size crowd - more so than Knott's on the same day. The park has a lot of open space, so it didn't feel very crowded, but there were some longish lines. Palm Springs is a smaller park, so the lines got long pretty early in the day. I haven't had a chance to try San Diego yet, but have been very pleased with the other two. They are run very well. With the exception of WDW, probably the best that I have ever been to.
Last weekend we went to Hurricane Harbor (with free passes) and it was everything that you expect from Six Flags. The park was overcrowded and the guests were very rude. We some guys actually eating in the lazy river! Given the choice between the two, I would definitely pick Soak City.