I think this is my first post...the M8 has made enough of an impression on me to do so.
History
I'm on Verizon, and am coming from a day one purchase of the Samsung Galaxy Nexus over 2 years ago...and (ugh) a Blackberry Tour 9630 before that.
I had my GNex rooted, and have been running AOKP most recently, with Gummy before that. I ran light on the custom side, not making too many modifications or other hacks. Mostly just worked within the functions the custom ROM provided, and would regularly download and use LeanKernel: Minimalistic Kernel and just change the governer (InteractiveX worked very well for my particular handset). Was very happy with the customizations the custom ROMs provided. Moving to AOKP 4.4.2 nightlies made HUGE differences in performance compared to 4.3 and before...everything was snappier/more responsive, and I noticed with the reduced resource/RAM usage that I could switch between at least one more app before it would have to do a full boot/refresh of that app. On to the M8. (Oh, and here's how I had my GNex's first 2 main screens set up...
...and here are the first 2 screens on the M8 now.
General
Everything [all the reviews are saying] is in line with my impressions. Everything, EVERYthing is immediately responsive...especially in comparison to the GNex. I have not found anything to be broken or having issues, whether they are apps or hardware. Build quality is great, though I'm protecting with the HTC Double Dip case (until the Dot View is in stock, at which point I'll likely switch). I guess if I have any complaints, it's that it really is a bit unwieldy in the hand...just a bit big and heavy, but those are things I can live with. The screen is beautiful. Coming from 720P, that's no surprise. Battery life is great for me in comparison to the GNex, which had many, many complaints about that. From a full 100% charge, it'll definitely last me one full day, though not much more than that.
Software/Sense 6
This is my first go-around with Sense, but I (amazingly, to myself) have no complaints. In comparison to running pretty light on AOKP, the general skin isn't obtrusive. I'm still trying out BlinkFeed, and am not sure on my final opinion of it as of yet. I normally use Feedly for RSS (and continue to do so), which I love...but I'm using BlinkFeed sometimes, and it may grow on me. If I were to miss anything, it would be greater control over how many rows/columns I can have on the homescreen, along with full control over size of widgets. You can maybe see on my pics how I have more flexibility on my old phone for this kind of thing. One other thing I miss from AOKP is the notification dropdown...I used the statusbar brightness slider and smaller/simpler traditional pulldown toggles. With Sense 6, I just keep brightness on auto and settle for having to use the dropdown toggle button first.
Camera/Software
The GNex had a 5 MP camera that didn't perform all that well. I made do, and generally didn't have any complaints. I'm not a photographer, so I don't set up the perfect shot with my phone so I can blow it up to 16" x 20" for my wall (I have a Canon 30D w/ good lens for that...but I STILL don't use that camera for that!). Given that, I'm finding that the general performance on the M8 exceeds my needs. Pictures look plenty clear for my needs...e.g. posting on Google+ and Instagram. I've tried a bit of Zoe, but mainly use rear camera in auto or HDR (haven't messed around with it enough to say if HDR mode is worth not having some of the fun after-effects). LOVE the UFocus effect. Darker room settings still have nice lighting. "Selfie" front camera has nice wide angle so people don't have to crowd next to each other to get in the frame. Etc., etc. Also, people don't think about disk space...16-20 MP shots or 4K video can quickly fill up the phone's space unless you obsessively manage it. 4 MP isn't so bad, thinking about it that way.
I find the gallery app (with those 30 second video highlights) to be amazing, though I wish I could integrate songs from Google Play Music with it. Generally extremely pleased with the camera, picture quality, and the software behind it.
Speakers/BoomSound
Very pleased with the sound quality coming from the front stereo speakers. Good volume for personal use or sharing with people standing right next to me.
BoomSound translates very well as the stock equalizer for me. I mounted my GNex in my car via the official GNex car mount, using the aux cord plugged into the head unit. I also installed a custom, unofficial Beats mod that had to be installed via custom recovery/bootload. The reason I did that is because the standard equalizers just didn't give that extra oomph on the low end. I have a small 10" sub in my trunk. My music is for me, not those in other cars with their windows down, so I keep it unobtrusive, but still found standard sound to not even translate the lowest frequencies to my sub. What I'm getting at is that BoomSound does that. Full sound, getting clear highs. Just wish the official HTC M8 car kit was in stock. Plan to get it when it is (just like the Dot View case).
Final Thoughts
Basically, I'm extremely happy with the phone. The low MP of the main camera is not an issue for me at this point, based on my usage. The build quality and look is superior (to me) to the plastic-y Galaxy S5, the small iPhone 5s, or the pictures I've seen of the Sony Xperia Z2. The Sense software does not detract from functionality or speed. I was concerned about signing away 2 more years of my life to another phone, but I feel that I made the perfect choice for my needs.