I feel the same OP, but I am going to give my iPhone 6 at least 2 weeks to see if I can adjust. I mentally can't go back because I like to have the latest gadget, so I really hope I can adjust.
I really do like how the 4 and 5 feel in the hand, I think it was the best design so far, and second nature when using them in my hand.
iPhone6 is slippery, I know there are cases, but I like my iPhones without cases, and never use cases.
Last night as I was driving and using GPS, I wanted to end the route and reaching my thumb over, phone slipped from my hands.
While I understand the need for bigger phones as there is a very large demand for it, I wish the iPhone6 was no large than 4.5in.