Well, I've had my One M8 for about 24 hours now, and here's my initial thoughts thus far:
1. Camera: this is actually a great camera. It's adapted very well to both low-light situations and high-light situations. A person sitting next to a bright window, for instance, came out very well. A flower at dusk also came out very well. I have no complaints so far with the camera.
2. OS/UI: I like the stock launcher, but I almost immediately replaced it with Nova. I love Nova so far. Very easy to use but has a kind of minimalist feel to it that I appreciate. Sense is closer to stock Android, but I like to try something new, and for me Nova is working very well.
3. Performance: No issues at all with performance. I had a software update as soon as I got the phone, which was downloaded (125MB) in about 2 minutes. It also installed in about 3 minutes. It shuts down almost instantly and turns back on so fast you could have it restart and not even notice. No lag, no performance problems whatsoever.
4. Battery life: This is the one area that I had expected a lot more than what I'm getting. It's not bad by any means, and I've currently had the screen on for 3+ straight hours and it's down to 36% battery (playing a fairly basic game called Game Dev Story) on top of some browsing and other things. A few phone calls, some texts, etc. Today is a lot more use than I would normally have, so it's not so terrible, but I had hoped for better battery life. I have no doubt it will get me through the day and then some with normal usage, though. So far, I'm either blind or my phone does not have the extreme battery saver mode. I'm on Verizon, which is probably why.
5. Build quality: This is flawless. No loose buttons at all. No glue coming undone under the screen. Nothing worth complaining about at all. It feels nice in the hand. It's not as slippery as I had thought it would be, and I'm going without a case for now because I want to enjoy this phone's natural beauty some more before I cover it.
6. Screen: This. Is. Amazing. That is all.
Overall: I absolutely love this phone. No regrets in my purchase whatsoever. Hopefully the battery life will improve as I play with it and when I eventually put a new ROM on there (thinking Clean ROM from a suggestion I got on here from buckifvr), so I'm not terribly worried about it right now. If anyone is on the fence about getting this phone, just do it! I felt the Galaxy S5 in my hand when I went to buy the phone and it just was not nearly as comfortable to hold. This phone is surprisingly light compared to my previous phone, and it's actually slimmer than I had thought it would be. Hardly feel it at all in my pocket, but coming from my previous phone, I could almost have an actual brick in my pocket and not notice it.
Could not be happier with this phone!