The days are shorter, the mosquitoes are fewer. And there is something about the light in autumn that just seems to make everything look better.
But it also requires a bit more awareness, all the way around. There are not so many people on the trails, so if you count on others for occasional help---you may wait a while in late September.
And the weather, which is always a factor in the Sierra, can turn downright nasty if you are not careful. A sudden snowfall will not only make things a lot colder and a lot harder to walk---it also makes the trail a lot harder to find. If you are not used to hiking in these conditions, then you better keep a close eye on the weather.
Even in summer, the weather can be unpredictable. On our two long trips this summer, we had hot weather one week, and three weeks later we were actually cold most of the time.
In August, at Summit Lake above Virginia Canyon, we found ourselves moving around the campsite during dinner, trying to find a spot that was in the sun and not too windy. It was about 50 degrees, and blowing hard. And it got even colder at night.
You can't really see it in the photo above, but the trees on the far side of the lake are whipping around in the breeze...
We've been in rain and snow in the Sierra in summer...so you expect that it could be even more dramatic in the autumn. The photo at left is above Carson Pass after a brief snowstorm left a dusting of snow on the peaks. It was enchanting. It was also only about two miles back to the trailhead.
Be careful, and have fun. And if the weather looks like it is going to change for the worse, don't leave any 10,000 foot passes between you and the car.