iOS 9 brought in some new good features. Don't just upgrade blindly, you have to know if your device is capable of running the latest or not. Before you read the review, I would like to tell u all that I will write the best and the worst iOS version of every device for people who are on iOS 7 and earlier to know whether upgrading to iOS 9 is a good idea or not.
First. iPhone
iPhone 4S
Best : iOS 6 / iOS 7
Worst : iOS 8
Upgrade advice : it's ok as there are no much new improvements to the experience and the performance, in fact my old iPhone 4 with iOS 7.1.2 ran better than an iPhone 4S with iOS 9 Beta.
If your are on iOS 7 on iPhone 4S stay there and don't even think of upgrading. If you are on iOS 8, the experience is quite similar.
iPhone 5
Best : iOS 7, iOS 8
Worst : N/A
Despite the beta bugs. It runs very good with iOS 9, you are sure going to feel it's a tiny bit sluggish, but that's ordinary as it occurs with most iOS devices.
iPhone 5C
Best : iOS 8
Worst : N/A
It runs iOS 9 perfect like the iPhone 5.
iPhone 5S
Best : iOS 8
Worst : N/A
It works very nice, there are a little bugs as it's still a beta.
iPhone 6/6 Plus
Best : iOS 8
Worst : N/A
Don't even think of upgrading or not, iOS 9 runs beautifully on those devices.
Second, iPad
iPad 2
Best : iOS 7, iOS 6
Worst : iOS 8
iOS 9 is indeed better on this device, it gives a chance for the iPad 2 to work again like it should. Sluggish animations are expected as the iPad is a little old but still better than iOS 8.
iPad 3 (the new iPad)
Best : iOS 7, iOS 6
Worst : iOS 8
iOS 9 will improve the iPad 3 like it should for the iPad 2, they both have nearly the same internals despite higher GPU on iPad 3. If you are on iOS 7, you don't need to upgrade neither to iOS 8 nor 9.
iPad 4 (iPad with Retina Display)
Best : iOS 7
Worst : iOS 8
iOS 9 is a life saver on the iPad 4, animations were a little sluggish and performance was bad with iOS 8. iOS 9 is a good cure for people stuck with iOS 8. If ur on iOS 7, don't think of upgrading.
iPad Air
Best : iOS 7 / iOS 8
Worst : N/A
iOS 9 is a bit slower but packs good feature inside. If you are on iOS 8, try the public beta first then think of upgrading, if you are on iOS 7, upgrade to iOS 8 or try the public beta to know if iOS 9 is suitable on your iPad or not.
iPad Air 2
Best : iOS 8
Worst : N/A
iOS 9 is excellent on this iPad, upgrade blindfolded.
iPad mini
Best : iOS 6
Worst : iOS 8
If you are on iOS 6 or 7, don't think of upgrading, if you are on iOS 8, upgrading to iOS 9 is a good idea as there are many performance improvements.
iPad mini 2
Best : iOS 7
Worst : iOS 8
If you are on iOS 7 or 8, try the public beta first.
iPad mini 3
Best : iOS 8 / iOS 9
Worst : N/A
iOS 9 runs ok, upgrading is a good idea as there are more features.
Third, iPod touch.
iPod touch 5
Best : iOS 6 / iOS 7
Worst : iOS 8
If you are on iOS 6 or 7, upgrading to iOS 9 won't be a good idea. iOS 7 and 6 run beautifully on this iPod.
iPod touch 6
Best : iOS 8
Worst : N/A
It's just released, its capabilities are perfect, it's expected to run iOS 9 very nice and sure without main bugs in the final version.
First. iPhone
iPhone 4S
Best : iOS 6 / iOS 7
Worst : iOS 8
Upgrade advice : it's ok as there are no much new improvements to the experience and the performance, in fact my old iPhone 4 with iOS 7.1.2 ran better than an iPhone 4S with iOS 9 Beta.
If your are on iOS 7 on iPhone 4S stay there and don't even think of upgrading. If you are on iOS 8, the experience is quite similar.
iPhone 5
Best : iOS 7, iOS 8
Worst : N/A
Despite the beta bugs. It runs very good with iOS 9, you are sure going to feel it's a tiny bit sluggish, but that's ordinary as it occurs with most iOS devices.
iPhone 5C
Best : iOS 8
Worst : N/A
It runs iOS 9 perfect like the iPhone 5.
iPhone 5S
Best : iOS 8
Worst : N/A
It works very nice, there are a little bugs as it's still a beta.
iPhone 6/6 Plus
Best : iOS 8
Worst : N/A
Don't even think of upgrading or not, iOS 9 runs beautifully on those devices.
Second, iPad
iPad 2
Best : iOS 7, iOS 6
Worst : iOS 8
iOS 9 is indeed better on this device, it gives a chance for the iPad 2 to work again like it should. Sluggish animations are expected as the iPad is a little old but still better than iOS 8.
iPad 3 (the new iPad)
Best : iOS 7, iOS 6
Worst : iOS 8
iOS 9 will improve the iPad 3 like it should for the iPad 2, they both have nearly the same internals despite higher GPU on iPad 3. If you are on iOS 7, you don't need to upgrade neither to iOS 8 nor 9.
iPad 4 (iPad with Retina Display)
Best : iOS 7
Worst : iOS 8
iOS 9 is a life saver on the iPad 4, animations were a little sluggish and performance was bad with iOS 8. iOS 9 is a good cure for people stuck with iOS 8. If ur on iOS 7, don't think of upgrading.
iPad Air
Best : iOS 7 / iOS 8
Worst : N/A
iOS 9 is a bit slower but packs good feature inside. If you are on iOS 8, try the public beta first then think of upgrading, if you are on iOS 7, upgrade to iOS 8 or try the public beta to know if iOS 9 is suitable on your iPad or not.
iPad Air 2
Best : iOS 8
Worst : N/A
iOS 9 is excellent on this iPad, upgrade blindfolded.
iPad mini
Best : iOS 6
Worst : iOS 8
If you are on iOS 6 or 7, don't think of upgrading, if you are on iOS 8, upgrading to iOS 9 is a good idea as there are many performance improvements.
iPad mini 2
Best : iOS 7
Worst : iOS 8
If you are on iOS 7 or 8, try the public beta first.
iPad mini 3
Best : iOS 8 / iOS 9
Worst : N/A
iOS 9 runs ok, upgrading is a good idea as there are more features.
Third, iPod touch.
iPod touch 5
Best : iOS 6 / iOS 7
Worst : iOS 8
If you are on iOS 6 or 7, upgrading to iOS 9 won't be a good idea. iOS 7 and 6 run beautifully on this iPod.
iPod touch 6
Best : iOS 8
Worst : N/A
It's just released, its capabilities are perfect, it's expected to run iOS 9 very nice and sure without main bugs in the final version.