My company is standardized on iPhone and several of us jumped the gun and installed the 9.0 update. Our corporate IT service desk sent out this email back on the 22nd:
A flaw in the new Apple 9.0 update has been identified that keeps the native calendar from syncing correctly. This can impact your ability to see calendar notices and reminders on the mobile device. Until Apple releases a patch for this flaw, and a few others that have been reported, we recommend adding Google Calendar for iOS to your device. While this may not offer all the functionality you are used to, it does reliably stay in sync with your company calendar.
I have 9.01 installed and this issue remains unresolved. I hope it gets fixed soon. What a pain....