I use an iPad mini 3 every day, 128 GB with cellular running iOS 10.2.1.
While it has slowed down a little bit, I still find it quite useful. All the apps I use on a daily basis work fine, no issues. I did disable some things like Siri suggestions (I don't use them), background refresh, and enabled reduce motion.
It works fine for me. I feel like this could be the last usable version of iOS for it.