Apple has proven that they are as fallible at releasing beta and poor quality OS and software updates as anyone else and I think the trend to wait until a couple of updates after the main update before upgrading your device is warranted these days for Apple.
If Apple wants to promote and brag about these kinds of adoption numbers they are going to have to step up big time and do a LOT more Q/A before the next major release. Not just on the Flagship phone or tablet they are about to release, but on any older supported phone as well. The idea of releasing iOS and then breaking something like phone functionality, or introducing a security hole is never acceptable.
For iOS 9, I will wait until at least iOS 9.2 or later, once the initial round of beta bugs get ironed out by the early adopters. I think this is the last time Apple will ever brag about OS adoption rate as they pretty much screwed the pooch on iOS 8 release and quickly created a market of skeptics.