Well, like everything new, sometimes it's best to wait. We are getting widespread reports of the iOS 10 upgrade causing iPhones to "brick" (get into the mode where they need to be restored), which is not good. Please hold off on the upgrade for now!
My iPhone 5s "test" machine and 12.9-inch iPad Pro received the upgrade with no problems, but my iPhone 6s Plus is one of the unlucky phones to get bricked. It's now seemingly stuck in restore mode, where I am trying to get it to restore the device to its previous state from an iTunes backup I made earlier today.
Dennis Sellers has also had an iPhone 6s Plus brick itself. Both of us also saw a message from iTunes saying that it "could not connect to the iTunes Music Library", although we're unsure what that's all about.
At least the batteries aren't blowing up...