Install It If You Use an APN
If you use an APN (Access Point Name) with your device, you'll want to consider installing Apple's new iOS 9.0.1 update right now.
The iOS 9.0.1 update comes with a fix that patches up an issue that caused some APN users to lose cellular data. Obviously, cellular data is extremely important (especially to iPhone users) so this is an extremely important fix.
We saw a number of complaints about APN issues after the iOS 9.0 release last week and it looks like iOS 9.0.1 does squash these initial problems. It's worth a shot if you've been feeling the effects of a headache in the aftermath of iOS 9.0.