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I have no major issues at the moment. I would say the only one thing that has happened to me occasionally is the swiping on the button to answer calls has sometimes let me down it has not answered the call as in there is no one there & the only option is to end the call. Has only happened a couple of times & I'm hoping it was me not the phone. I'm pretty impressed though.
 
There is still a lot of room for improvement on iOS 8.1. Probably even more so with the 6+, however I don't have one so I can't say for certain.

People can say otherwise, but there are clearly some optimization issues with the 6+.

Same thing happened with iOS7 and iPads. There was really sluggish animations and transitions and it wasn't sorted until iOS 7.1
 
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