Use it everyday!! I use it on my iPad 4 and iPhone 4S. With that said here is an article I wrote a few weeks ago about iOS that points out good key points about why 6 and older were better than the newer ones.
Today while I had some free time in Biology, I decided to look at the iPad Pro as possibly my new iPad. I scrolled and looked at the previews then I got to the part about iOS 9. It says "A better experience with every touch. iOS 9 is the world’s most advanced, intuitive, and secure mobile operating system. From powerful multitasking features to Night Shift and News, iOS 9 is designed to help you get the most out of iPad." I was one inch away from tipping over in my chair laughing. iOS used to be able to hold its ground like Apple said before iOS 7, but now you just can't say that iOS is better than iOS 6 and older. The experience is dead and Apple is struggling like crazy trying to optimize it to run better.
Ever since Scott Forstall was fired from supposed "complications" working with him, which I believe was him not wanting to get rid of the graphical design he made for OS X and iOS with Steve Jobs in exchange for a flat design. Everyone else says it was from him not apologizing for Apple maps which I can't confirm because I've never talked to him and I live on the other side of the country. I've heard many things about why he was fired so I really can't say too much. Anyway, Jonathan Ive has taken over software design and now manages both Industrial and Graphical design. With Ive now in charge, we were given iOS 7 with a new design and an over simplified UI. Now every app you open, you are blinded with a white UI that looks like it's fresh out of the 80s with app icons that barely match the content of the app at all. Ive said he wanted to contain consistency and that skeuomorphic real looking textures were no longer necessary as for he said everyone knows how to use their iDevice now. What he didn't realize is that skeuomorphism contains consistency but allows for different user interfaces to tie together at the same time by using the same style of design. He also didn't realize that pretty much everyone liked the old design and didn't care about a change to a flat design. Most people that I talked to after iOS 7 came out said that they wish they could go back because they absolutely hated it. I went back and I didn't regret it at all. I still use iOS 6 every day on my main iPad and my old iPhone 4S.
Have you ever wondered why the nice finger tips quick start guide is no longer included with iDevices, and all you get is a piece of paper telling you what the buttons do? Its because there is absolutely nothing to show off in iOS. It's almost embarrassing how crummy the UI looks. They can't really show how an app looks and functions because THEY ALL LOOK THE SAME!!!!!!!!!!!!!!!!!!!!!!!!!!
Wondering why your iDevice is running slowly, well I believe I have an answer for that. iOS 7 and up uses stylesheets much like CSS files for websites. The processor has to generate the UI from the ground up as soon as you open an app and that also applies to notification center, control center, and spotlight search, but you don't have to worry about those because iOS generates the style for those when your booted into iOS.
iOS 6 and older didn't use stylesheets in that fashion. iOS was image based and made the UI much like layers. The navigation bars, background, buttons, etc, were cropped and put into the appropriate spots via a stylesheet pointing out where they are supposed to be.
On a moderately powered device, having to generate the UI from the ground up takes a HUGE chunk out of performance. It's ridiculous, my iPhone 4S almost five years old on iOS 6 can perform just as well as my new iPhone 6S.
So what can be concluded here? Well iOS has become a lifeless, unprofessional looking, goofy, displeasing, buggy, laggy, and clunky operating system with too much blinding white UIs and pointless and obtrusive Gaussian blurs that only bog down your device. Jony Ive just doesn't understand that you can't just load down the processor and expect good performance, even on a more powerful device. He makes great device designs (except for the iPhone 6), but he can't do graphical design all to well... or at least efficiently.
Seriously though...
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Hmm... What a shame...