I am interested also. The best season should be from say may-oct with Sep-Oct the peak season as lots of visitors wanted to see jiuzhaigou with it's falling yellow leaves.
Make sure u avoid the Golden week from 1 Oct and maybe a week before and after. So the best is to go during Mid Sep or Mid Oct. Usually there will be photography tours organised by the chinese photo mags in their Aug-Oct issue... i think those should be good.
If you join a non-photog tour, i think u will missed lots of things and has no chance to wait for the right lighting, use tripod etc...
Not really cold in mid Sep. It all depends on individual tolerance to cold and the change in attitudes from one part of Jiuzhaigou to another. Just bring along a jacket/sweater and use it if you feel cold. That's what the tour guide told us to do during my trip.
Besides that, you might also want to take into consideration the possibility of attitude sickness. Some people are very sensitive to sudden changes in attitudes and can become very sick. Those who suffer from migraines might have a problem. A lady in our tour group was so sick that she had migraine attacks and threw up during the whole journey up and down the mountains and could not join us on the walks and climbs inside Jiuzhaigou ! Don't think she would be planning on any more tours to high attitude places after this experience.
I just got back from various parts of Sichuan, including Jiuzhaigou.
Now's the best time to go. However, it gets quite cold in the higher mountainous regions like Huanglong, Hailuogou etc.
This is the best time, if you want to experience snow, red autumn leaves all at once. Be prepared that you may not get nice blue skies with white fluffy clouds since weather is quite cloudy and foggy.
I went on a customised package tour where we formed our own group. The bus rides to certain areas are quite dangerous because roads are not fully developed, especially on the way up to Hailuogou where there are landslides galore. In these areas, be prepared to be stuck in jams for up to 4 hours.
No problems with roads leading into Jiuzhaigou. If you want a faster way to get there, catch a domestic flight from Chengdu. Flight time abt 40 mins vs bus ride abt 9-10 hours.