Beta 4 for me is quite good on both iPad Air and iPhone 5S. I tried beta 1, had to give up on beta 2 and now upon returning to beta 4 I'm more than happy with it.
On both devices it is stable, had a few odd screen issues with orientation, but nothing that stopped me from doing what I needed.
The killer feature for me is the ability to fire up hotspot on my MacBook Air without having to take my iPad out of my bag and fiddle with settings, it is truely fantastic. I've now dropped using mailbox (private email) and boxer (work exchange) in favour of the great new mail app, especially with swipe actions and contact completion suggestions, obviously both were in beta 1 however the general platform was a struggle.
Battery life seems on par with 7.1.2, so happy with that, surprised actually.
So far happy with beta 4, no thoughts of heading back to v7.x and enjoying the new features.