I have had the M8 since the day after it came out on Verizon in March. When I first got the phone, I enabled the setting that tells me what "percentage" of my battery is left. It freaked me out because I felt like my battery was dropping too rapidly based on the use. However, I think it is because the percentage meter is not the most accurate and it also creates some kind of mental insecurity with the user that breeds fear that the battery will not last long. I finally shut that setting off and just relied on looking at the little picture of the battery to tell me how much life I had left. For the past two months, I have never freaked out about the battery percentage and realized that my HTC One M8 lasts me from 6 a.m. when I wake up until 11 p.m. or so when I go to bed. I have not had it run out of battery on me yet.
I use my HTC One M8 all throughout the day. I often finish writing motions on Google Docs while I am sitting in court for case managements. Why am I telling you this? Because you shouldn't sweat the battery life. You should be getting all day battery life out of your M8. I don't play games-- my usage is email, email, more email, drafting documents in Google Docs, reviewing discovery on Google Drive, Facebook, and a lot of text and Google Hangouts messaging.
As a former iPhone user as well, I can tell you that the M8 has far superior battery life. Although, it is not exactly a fair comparison since the iPhone battery is a lot smaller. The iPhone does great for how small of a battery it has.
If you are not experiencing all day battery life with moderate usage, then you may have an issue.