We made our first ever trips to Kings Island last year and both visits were on Saturdays. One in mid May, the other in early August. We waited until we got to the park to buy Fast Lane Plus, it was well worth the money, especially driving 5 hours to get there! Even if a line was only 15-30min long, it was pretty much a walk on for us. Having that be our wait to get on rides, it was a great experience!
The longest waits were for the coasters obviously (45+ minutes), but those were pretty much a station wait to 5 minutes with FL+, depending on what seat you wanted. I think we did a mini marathon of The Beast, Banshee, and Diamondback. The looks we got were bad, but if we are there for just the day and they don't put a limit on it, might as well do it!