A few days ago, I warned users of Office365 and Outlook.com that they can't update to iOS11 due to a glitch that would prevent them from sending or replying to emails. Last night Apple released an update to iOS (version 11.0.1) which fixes this glitch, along with a few other issues. From a technical standpoint, it is now safe to update to iOS11, so long as it is version 11.0.1.
Please keep in mind that if you use older Apps, they may no longer function in iOS11, so open your most important apps before upgrading. If they are incompatible you'll receive a message that says the app will not be compatible with iOS 11. For example, the old game Flappy Bird will not run in iOS11.