I'm have iPhone 6 less than a month, and iPad Air 2 less than a week. This is the period where I least enjoyed iOS.
My iPad is very buggy, especially video playing from Safari and multitasking gestures. Every day the multitasking gestures would fail, EVERY-SINGLE-DAY. You're using iPad normally then bam! pinch to Home Screen not working. And there's no way to fix it apart from restarting.
..which you'll lose Touch ID capability (you need to enter the passwords again - password for log-in, password in the Stores)
Playing video from Safari also is very buggy. Sometimes the control is not responsive, or simply stuck. You can quit Safari and re-open and the video player would behave normally again but just for a while. Sometimes when you tap "Done", nothing happened. You can only re-start to fix this. And then you'd need to re-enter all the passwords again.
At the moment, I simply turn off Multi-Fingers Gestures.
I never experienced this level of bugginess before. iOS's (4-6) always smooth and most reliable to me. But I never restored from back-up before and this is the first time. This might be the cause.
My iPhone 6, however, began as new and it's a smooth sailing.. until today. Today I woke up to find all my Active Calories from Strava, but the latest one, was gone. I can re-input them manually since Strava kept all the records online but this shouldn't happening, especially on Health. Think about one day your important Health datas just simply disappear. What a horror!
If there's any app that should be the most reliable, it's Health!
This is the worst experience I have from iOS. I had iPhone for a short while. It's on iOS 4-5. I had iPad 2 until a few months ago and mostly it's on iOS 4-6 and it's always rock solid. If Apple not get the acts together very soon they can kiss goodbye those Customer Sat stat they love to brag about on stage.
:end rants: