Oddly enough I am wearing my 33 year old Yosemite tee-shirt. I have been twice, climbing and back packing in the 80s and 90s. Although busy it is very easy to get to places where you are soon on your own. At least you could do back in the day in September.
We've been twice too. Both last year. In the "Fall", the valley was busy and the traffic was terrible. But we saw it as just having to suck it up. For a lot of people cruising around the valley loop in a vehicle is the only chance they'll ever get to see scenery like that. There was one day we wanted to do Pothole Dome but simply couldn't get there due to the traffic heading up to Glacier Point (I think!). But we still managed to find plenty of peace and quiet just a short walk from the crowds. It was also clear that 90% of them stayed in the valley. We spent an incredible evening watching sunset at Tenaya Lake completely alone. Bliss.
However, our first visit was mid February. There was literally NO ONE in pretty much all the areas we visited. It was utterly incredible to be in such a stunning winter wonderland and the vast majority of time not seeing any other humans. Vernal Falls was the only busy bit and we learned later that it's the most popular walk in the Valley so I guess it's to be expected.
A winter visit is so very worth it.
All that said, the surrounding area of the Eastern Sierras is probably even better, free, and far less busy. I'm not sure we'll be back any time soon even though we'll be passing right by it.
Photo from our memorable night at Tenaya Lake, elev. 8,150ft, just because: