I've been testing 9.1 beta for the past month or so, just put my iPhone 6+ and iPad Mini on production iOS 9.1 and have found no new bugs related to iOS 9.1 so far.
I usually like to recommend waiting a week or two after a iOS release. I feel it's a safe upgrade, however if you have a critical flight, I'd hold off just to be on the safer side. The iOS 9.1 release fixed many bugs with my devices outside of iFly GPS, and so far I can't see any negative impact of iOS 9.1 on the iFly GPS app.
As always, upgrade at your own risk. As each device and installed apps and can react differently.