Instead of me sharing my experience which every other Apple user is against, instead of making threads like these; please do some research of your own.
This includes reading the IOS 9 sub forum and go watch the countless YouTube videos of any IOS 8.x version compared to IOS 9.x version.
My stance why install to get only 3 new features with a laggy/crappier keyboard in an new version of a OS with so many glitches and bugs and core functionality not working 100 percent.
When if you stay with your current OS version you have 100 percent functionality and can continue to use your device the way it was intended to.
One other thing, upgrading to IOS 9 might work out great for you, most apps on AppStore are still only optimized up to IOS 8 and a lot of them have not been worked on yet to be optimized up to IOS 9.
This means one or more second delay in IOS 9 when touch clicking anything before it even begins to load. Once it loads some apps can take up to 30 seconds to load due to them not working along with IOS 9. Also crashes and freezes galore.
I say stick with IOS 8.x as long as you can.
Do not take my word for it, research it, come to your own conclusions.
Once you upgrade, you cannot downgrade and Apple support will be of no help.
i really love my Air 2's performance, but I want the ios9 features. I keep reading about lagging when updating and I would hate to have the same frustration I get when using my 6+.