Just an update on my latest iPad state. I think it is good news!
My first of iOS 9 installation ended with the iPad unable to boot up after being powered off. I restored to iOS 9 on my iPad as previously described (first install as setting up a new iPad, then apply my saved backup). I did that on Wednesday. But, I was still noticing the occasional hanging Apple logo screen. I did not try powering off to see if I could get back. Before just going back to iOS 8.4.1, I discovered the newly-released iOS 9.0.1 which came out on Wednesday. I figured I had nothing to lose by installing it, since I could probably still revert to iOS 8.4.1 if needed. I am happy to report that all the hanging Apple logo screen problems are gone with this update, I have powered off and on just fine, I have also done a hard restart just fine. The iPad seems to be running more smoothly, too.
I then encountered nasty "Waiting..." app problems since then. I would delete and reload individual apps, but then the Apple App Store gods would act and lots of apps would go back to "Waiting..." state with no way I could find to get them out of it from my iPad. I just synced all the Apps via iTunes on my computer and they are now all out of "Waiting..." state. Hope it stays that way.
I also have now taken the advice of others here and saved my iPad backup from before I started this process in another location on my Windows computer. It was not overwritten when I upgraded to iOS9 by asking to set up a new machine. Guess I was lucky, and I would prefer to not test that again.