Well, my iPhone SE, still on 9.3.5, will probably be updated to iOS 10 in a few months, when the initial wave of bugs and inconsistencies is gone. Besides, some of the new flag features (like the new iMessage, the new Notification Center / Widgets ) didn't call my atention.
I'm not one of those that prefer to stick to old versions just for the sake of it, specially with all the security fixes that keep on coming on current versions. It's just that, in my iPhone 4 days, I got a little burned (is it correct to say that? 😛 ) for the huge performance hit it took after iOS 7. After that, I tend to think that 2 iOS upgrades are safe for one kind of device, but the third... well.. That's why my other iPhone, a 5S, will most certainly stay on 9.3.5, even if some tech sites keep on saying iOS 10 is fine even in iPhone 5 / 5C hardware..