I work on phones (mostly iphones, but some androids as well) and 9.0 so far has been the greatest dichotomy of OS updates I've ever seen. Half the people love it, and the other half of the people are getting completely unresponsive touch screens, outright bricked phones, update freezes, and dozens of issues that apple is either ignoring and telling people to upgrade their phones to fix instead of admitting fault or just denying outright (kinda like the error code for a screwed up home button not downloading updates that they pretend doesn't exist.)
I'm not saying 9.0 beta is bad, the people who it works for love it and it seems to be the best things I've seen in a while from apple. But the people it doesn't work for essentially get nothing but a chunk of metal for apple trying to beta test their software on devices never meant to run it.
Essentially, if you plan to get it, just do what you should be doing anyway with every software update and make for damn sure you back your phone up, cause if you have anything older than a 6, its a decent chance you'll have some issue with it. I've yet to see a phone that a simple restore didn't fix every problem that 9.0b caused.