Have the 6 Plus. The vibration is solid and appreciated, but I personally wouldn't say it's overly noisy. I have noticed some of the unoptimized apps, but I imagine that will be cleared up shortly with some updates. Now with regards to size, I love the size of the screen as I previously owned a Note II. I also love how much easier it is to see out in the sun and hope they bring the updated polarizer to the iPad's. That being said, my only complaint would be the size of the bezel on the top and bottom of the phone as they seem excessively large. Other than that, love the phone AND I love the fit/finish of the silicon Apple case. A little pricey, but worth it for the quality and super slim fit. Had a Samsung TPU case for the Note II, and it was simply ok.
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Nope.
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We will see the effect of fragmentation in Apple ecosystem itself which once it abhorred !
Nope.