I've avoided using Apple beta software for the past few years after being burned several times with buggy software. It's a beta for a reason, and I use my devices far too much for work to take a risk of losing data or productivity.
This being said, today I installed the iOS 9 beta on daily iPhone 6 Plus.
iOS 8 has been far too buggy for me lately to not give iOS 9 a chance - and so far it's been stable - far more stable than 8.4 was for me.
I won't publish any thoughts on iOS 9 until it leaves beta. For now, you can rest knowing it appears to be a much more stable release than iOS 8 was.