Been using it since 5pm yesterday, here are my immediate opinions. For reference, I currently use the Lumia 920 and Sony Z1 Compact, and I previously owned the original HTC One M7, so nothing here is new to me per se...just the combination
The build quality is, as you'd expect, awesome. The phone is very well built, feels good in the hand, etc. That said, I do find it to be way too big. I am a big fan of small phones (as evidenced by my Z1 Compact ownership), but it's a sad fact that you just can't find a small sized Windows Phone that's also high quality. One handed usage of the One M8 is nearly impossible for me.
Software-wise, it's Windows Phone. You know what you're getting here. Everything works very fast and fluoid (although BlinkFeed is a bit jittery). It's faster than my Lumia 920, but I'd sure hope so, since the 920 is almost two years old. Double tap to power on works well, but it's not the same as on the Lumia - you have to pick up the phone first. So if you (like me) leave your phone out on your desk most of the day, a double tap won't open it on its own. The LED is present, but like previous HTC Windows Phones, only lights up when it's charging, not for notifications. That's unfortunate, considering the APIs seem to be there (see Glance on Lumia), and the hardware is there...why not put it together?
Haven't used the camera too much, but it's really not as bad as people make it out to be. No, it's not Lumia quality, but would you expect it to be? It's on par or better than the Z1 Compact, which is impressive given the sensor in that phone. One positive is that it's very fast. Sometimes it feels like my Lumia takes a year to focus and snap a picture - the One M8 launches, focuses, and takes a picture very quickly, which is welcome. I definitely miss a hardware camera button, but if you add the camera to the action center, you can still launch relatively quickly.
Too soon to really have an opinion on battery life, but for what it's worth, it's been off the charged since 5am and is currently at 74%. According to my ol' pal Battery Widget that's good for a burn rate of 3.7% per hour, which is reasonable. It has more remaining than my Z1 does, despite having more usage, for what its worth.
Last note, Verizon LTE is terrible. My other phone is T-Mobile, which is where I'd be switching from. I get 21 down on T-Mobile LTE, 9 down on T-Mobile HSPA+. Meanwhile, I'm getting 2.9 down on Verizon (yet to see higher than 8 anywhere). Pretty terrible. I might be better buying this full price and using it on HSPA+ on T-Mobile...
So far, I like it, but the combination of it being bigger than I'd prefer and Verizon being terrible is giving me a bit of pause. We'll see.