Switched from Verizon/Galaxy S3 to AT&T and ordered the M8. I've had it for 5 days now.
Build: It's a beautiful phone. It's elegantly sturdy. The screen has been great so far. I personally love the weight of the phone as it makes it feel more fuller. It's my first time have a non-plastic cased phone (Incredible, Droid MAXX, S3) and it feels pretty great in the hand. With that said, I still feel like it'll break/crack easily even with a slight drop. I've ordered both a case and screen protector off Amazon from Red Shield until more options come out.
Speed: The phone operates quick and smooth. Haven't experienced any lag so far.
Sound: worlds better than the S3. the speakers on the phone itself is definitely an improvement from galaxy. First phone that I've had where the speakers are on the front and sound is crisper. I love listening to my own music when I drive. Whenever I had my S3 plugged into the AUX port (or any AUX port) it seemed like I'd still have to crank the volumes on both the phone and car stereo all the way up and sometimes even then it'd still sound too quiet. having the M8 plugged into an AUX port is much better where volumes on both the phone and AUX port don't have to be cranked up all the way.
Display: Haven't played any games really as I'm just trying to find out all the other features of the phone, but pictures and videos are crisp. My only gripe is the reasoning behind putting an HTC label under the screen towards the bottom bevel (especially considering the HTC label on the back is nicely put). that could've easily been more screen room or even where the pop up keys appear.
Camera: I've only taken a few pictures so far. Haven't tried to utilize all the different camera modes. I've yet to try and take pics in low light situations. I understand why a majority of people say the camera is a deal breaker when complaining about megapixels. But for me, I'm no photographer and I'm not taking pictures to make a portfolio or anything. I'm just trying to pictures of moments that I like that I want to save/share later. With the few pictures I've taken, I've barely noticed a difference compared to pics I took with the S3.
Apps/Feat./Misc: Honestly, I wasn't sure how to get the most out of this phone which is why I decided to make an account here. Right now, I'm using the M8 for basic use- social networking, watching videos, browsing the interwebs, etc.
I hope this was enough for average users like myself. I've still got till next week to change my mind on the phone so I'll update if needed.