Some people may have trouble with the UI
But, if a UI is superior whilst requiring a steeper learning curve, it is still superior and worth the effort.
I bought one of the first PalmPilots and spent an age playing Giraffe to learn Graffiti. After the initial couple of hours I was a whizz at inputting using the stylus - fast and useful.
Likewise, WinPho, lovely interface, easy to use, smooth and elegant.
I do not see this slow down or battery drain some have mentioned here (60% remaining without a boost today on a two year old 1020).
I am concerned that Win10 may not be as nice to look at but there may also be some gains with the sacrifices (although I don't really need a word-style editor for my emails, perhaps having all the attachment capability would be useful).
And the possibility that more apps will appear, matching the Surface/PC apps adds to the allure.