It isn't great on my iPad 3, though 8.0.2 has seemed to help a bit.
It runs fantastic on my iPhone 6, my wife's iPhone 5s... And decently enough on my old iPhone 5 (which is better by far than the iPad 3, because RAM, I think).
They'll improve it over time. The iPhone 4 sucked hard on iOS 7 at first, but is fine now on the latest builds.
I have an Air and I think this is the tipping point for me to finally ditch the iPad, and possibly iOS. It's been an unbelievably poor experience, with constant issues in the majority of apps that I use.
Safari is worse than ever before and is painfully slow to render pages now.
2/3 of the time I have to manually refresh pages as I scroll down them because they are just blank.
I had been considering a new iPhone even though the specs are not what I'd like (ideally 3GB+ RAM, 256GB storage, MicroSD expansion) but there's no way I'd consider one now. The main issue I have is that some of my contacts
only use iMessage now. I can't believe that they have a closed messaging platform in 2014, or that people accept it.