I use it on my iPhone 5 which i bought off eBay for 50 $. It had 10.3.4 installed but i jailbreaked it and thanks to the CoolBooter tweak i can mount a second partition to the phone and install any released firmware (iOS 6,7,8,9 or 10). I never thought that such a thing would be possible. It's amazing. That's the reason i bought it, for pure nostalgia. So i downgraded it back to 6.1.4. And man, the iPhone 5 is so fast on iOS 6. I don't need it though, my daily driver is the 8 Plus on the latest 12.4.1, but for 50 $ i couldn't say no. It even came with the original box and the included EarPods are still sealed. And the documentation still has this paper smell. This thing is nearly 7 years old, how time flies.
It really brings back memories, my tech journey with Apple started with the iPhone 5 and iOS 6. I mostly use it as an iPod. The audio quality is superb thanks to the headphone jack. And i can load old Apps and Games to it that weren't supported with iOS 11 anymore, also because i still have access to my old Apple ID that i used back then.
iOS 6 itself works fine, just a couple things don't work anymore like the stock weather application. And you have to set the timezone yourself, it can't automatically fetch the correct time & date anymore. Apple must have switched providers for those services, i know they don't use Yahoo! Weather anymore.
And if i really need to use it as a backup phone one day, i just boot the latest 10.3.4 firmware and most apps still work with that. Usually i don't jailbreak, but since Apple doesn't sign older firmwares, that was the only way to restore to 6.1.4. Well Apple is still signing 6.1.3 for the iPhone 4s and iPad 2 for whatever reason, so you can downgrade those two devices directly, but not the iPhone 5.