iOS 9 download is available for quite a long time, so if you are wondering if you should upgrade your iOS8, here are the most important changes:
iOS 8 vs iOS 9: Siri
Apple says Siri in iOS 9 is 40% more faster and powerful than its predecessor (making searches quicker). Also, now Siri will understand more, because of the contextual awarness.
iOS vs iOS 9: search
iOS 9 has some new features like swiping left from the home screen launching a search splash screen, filled with suggestions from Siri.
iOS vs iOS 9: battery
In iOS 9 we have access into a new low-power mode which drops the screen brightness, lowers the frequency of downloading notifications and messages, thus lowering the usage of device's battery.
iOS vs iOS 9: multi-tasking
Several options are now available for those wishing to multi-task on their tablets, split-screen apps, the two apps running side-by-side simultaneously.