Long post - but I hope you like it.
I recently switched to Verizon from my T-Mobile Prepaid plan. I have the HTC One M8. I like the phone, but couple of things to note:
Unlocking the bootloader on Verizon's One M8 may be tricky if the phone has been updated to 4.4.3. Last time I checked, SunShine is the only available method, and it's $25 per device.
http://forum.xda-developers.com/showthread.php?t=2792487
If on 4.4.2, it's pretty easy to unlock & root.
Verizon has removed the default FM radio app. I found the apk with googling, and NextRadio on the App store will work as well. I still have the file - If you need it just send me a PM.
Camera is sub-par, especially when compared to iPhones and other high end android phones. It's a 4.1MP on the back, I believe. 5MP on the front is decent enough. Back camera's good enough for social media and quick snaps, but blow them up on a desktop or for printing and they look quite mediocre due to the low MP count. Some pictures I took for /r/pebble: http://imgur.com/a/A4O6s
Back camera also has reports of being easily scratched - so much so that picture quality can be affected. Toothpaste rubbing is supposed to help fix this, though, and I haven't noticed this problem with mine yet.
Sense 6 on my One M8 doesn't allow me to remove lockscreen factory reset. I had to root, s-off, and unlock the bootloader to change a number in some system file.
http://forum.xda-developers.com/verizon-htc-one-m8/help/removing-htc-one-m8-lock-screen-pw-reset-t2880459/post55521850#post55521850
Youtube glitches up sometimes - the status bar and navigation buttons don't fully go away. Black bars stay. I'm not the only one with this problem, but others don't have it. It also happens on other android phones as well. I have to use "All in one gestures" for "immersive mode" which fixes the problem temporarily. I have to restart the YouTube app to fix the problem.
Front facing speakers are awesome. I thought they were overhyped when I first tried them out, but they've either broken in or something, because when I try my dad's iPhone 6, the little speaker at the bottom sounds pathetic now.
Pretty long battery life, especially compared to my old iPod Touch 5 that used to be my "smart" device. I can go through plenty of long YouTube videos before having to recharge. I tend to have at least 20% battery at the end of a heavy day. WiFi or Data is always on, bluetooth is on to connect to my Pebble, sometimes GPS to get to new places around college, and lots of web-browsing on my 2 hour bus ride to college and back. I have never had my phone shut down due to a low battery. Ultra low power mode is also available on Verizon with the 4.4.3 update.
Verizon's One M8 has a lot of bloatware with Sense 6. Fixable with "App Quarantine", but if you flash stock android, that shouldn't be a problem.
Power button is a bit hard to reach in the beginning. Volume buttons protrude out more than I'd like.
Heavy compared to the iPhone/iPod Touch. If you like to hold your phone to watch YouTube videos, your arm may get a little workout at the same time.
These are all most of the things I wish I knew before I bought my One M8. After finding solutions to most of the major drawbacks, I'm quite happy with the phone overall.
Also, Verizon's CDMA, so a One Plus One would indeed not work, unfortunately.