Unlocked or not, my curiosity on other's experience on this issue - for those buying an unlocked phone from MS, do you think the firmware updates still come through on an unlocked phone that is connected to AT&T? My experience is that the cell provider locks the firmware updates going through its system. I have an unlocked phone, connected to AT&T with an AT&T SIM card in it and I don't have Denim... but the same model everywhere else has Denim. I had to do Preview for Developers to get the latest WP 8.1 software updates too.
I have no idea how the Windows 10 mobile updates will happen. Based on my experience with my current phone, I am concerned that it will be same with the new phones - if the phone is connected to AT&T's network you are still at their mercy for firmware updates (because Microsoft doesn't control the experience on their phones - the carrier still does), even if the phone is unlocked and bought from MS. Why buy an unlocked phone at a premium, if AT&T sells for less? I could buy the phone for full price from AT&T (when they start to discount they usually go down about $50 to $100 within a month on phones that don't sell), ask for the unlock code (so I could use on trips to Europe) and would have the same (lesser) experience as I do now but pay less.