Day 2 update:
After seeming to have sub par battery life, I decided to do some tests myself. Im not a heavy user, with most things set to manually sync. Weather refresh in 1 hour, some texting, and phone calls. Yesterday, After about 12 hours of time off charger, I was at around 70% battery, with 4 mins of Angry Birds, 30 mins of phone calls, a few texts, email, and about 45 mins screen on time. I decided to throw the Power hammer at this thing, and turned on the WiFi, and played a 50 min Pink Floyd video at 50% screen time. Then, I loaded up an 1 hour and 30 min Van Halen concert, and bumped the display to 75% brightness. During the video, I had to field more calls, so I had to pause it from time to time. Video ended, so I throw some more Angry Birds at it. Anyway......After 16.5 hours off the charger, 4 hours and 15 mins screen on time, 1 hour of phone calls, and some misc light use, the phone was FINNALY at 8% battery life. In comparison to my MAXX HD, 1 hour of calls, light use, and 2.5 hours of screen time, would have put the battery at 25%. The MAXX HD has GREAT standby time, and easily goes 2 days off the charger with light use. However, me thinks the HTC One M8 has a much better battery life when actually being used.
Interesting thing to mention......yesterday at this time, 3 hours off charger, I was down about 10% with very very little use. Today, Im only down 3% with the same use.