I am really enjoying using my HTC One M8 for Windows, and I must say that the battery life on this phone is quite impressive. I have used the phone moderately today, and it is still at 80%, after 14 hours, quite amazing.
I am very impressed as well. I originally thought the battery was just meh, but after a few days of proper charge/discharge I found that after 15-16 hours of regular use I still have about 15-20 percent left. All of my other phones needed to hit the charger after about 8 hours with the same amount of usage.