I have 8.4 on my 6+ and have seen no issues with it so far. I have it on my iPad3 and I think I'm seeing some level of improvement. There have been no page reloads in Safari yet, and that is progress in my book. Javascript still runs like a pig, though. The WiFi seems a little laggy, too, but maybe that's just my impatience with IOS8 on my iPad3 at work. But overall, the stability seems better.