Last week I switched to using a Lumia 640 XL as my primary phone, driven by the issues with the Lumia 930's microphone array and the damage a family member had inflicted on their Lumia 735 which meant that I had to exchange it for my test Lumia 735.
First thing I did was install Windows 10 Mobile onto it, so one comparison I can't make is battery performance between the two Windows versions. However in my first few days of testing I was very impressed with the longevity of the 640 XL on Windows 10.
This weekend I set out to see just how good that battery life is. Leaving my office at 3pm on Friday I unplugged from the mains and used it as normal. My only concession to extending the battery life was enabling Battery Saver to turn on automatically when 15% battery remained. This stops all background tasks except those which you give permission to continue to run in the background.
This morning I arrived in the office at 8am with 5% of that charge still remaining. That's better than two and a half days on a single charge. Hugely impressive, when I normally count the ability to go from one morning to the next evening as excellent battery life. I imagine if I'd manually enabled Battery Saver overnight then three days would have been easily achievable.