Never be the first to upgrade software or buy new hardware. It’s a good rule of thumb that’s helped me avoid problems like this. Personally, I usually wait a week, watch the tech websites and then decide if it’s the right time. I’m sure I will upgrade to iOS 9. It will just be a matter of timing.
Also, back up your device before upgrading, so if the worst happens you don’t lose everything. If you can, perform your upgrade via iTunes – not wifi. In my experience, choppy or poor wifi connections can lead to upgrade grief.
Has there been an update that I regret? Well yes, upgrading my mini2 to iOS 8 was really a backwards step in terms of performance. And subsequent updates to 8.4 etc. didn’t solve the performance problems. But I knew it was risky when I did it. But I wanted to see the accessibility features (which were an improvement) in action.
In fact, my mini might be the first device that I do upgrade to iOS 9. Since I don’t have much to lose at this point, it might be a good way to check out the backwards compatibility of iOS 9. In theory with iOS 9 Apple is doing more to ensure that older devices will continue to perform well. I’ll let you know.