[One (M8) Original Android Development] CutGlass:
What you need to know - if something fails and you think you need to run the battery down because it's stuck. Holding the volume up and power key for a long time will black screen hard reboot it and holding the power and volume down gets into recovery.

However there is a choice to be made. With your stock ROM your end up with ugly red text saying stuff about htc accepting no liability blah blah... and this is because the device is S-ON which means though you can install ROMs certain things can't modified, "root" is not really "root" on an HTC device. Now with S-OFF you can change the red text. To do this means paying some former (possibly current it was a bit controversial charging for an item) http://theroot.ninja for an app called sunshine that only works with HTC stock kernels. The app is free if it can unlock the phone if it's running a stock HTC kernel and will only ask for payment if it is possible. Anyway it works out about $25 USD and there's a thread on xda about then changing the boot loader screen and removing the red text for free.

Now if you have used sunshine then the step below may not be advised as the latest firmware might revert the phone to S-ON again. (have a chat with them if you decide about the text).

Use HTC sync on your PC to make sure your phone has the latest firmware as indeed this has probably been sitting in a warehouse. Reboot and just check it's working first. Go to http://www.htcdev.com/bootloader/ to unlock the boot loader. I would install fast boot flash TWRP recovery myself but there are issues at the moment with the lineageOS updater not running the scripts to reboot and install automatically (it downloads a file to /data/data/org.lineageOS.updater/app_data which you can find in TWRP (and you will ask about) when it updates next week and saves you having to install (another tip is to delete the file then in the updater which you get to in About phone, long press on the download you want to delete).

So far no-one I know apart from maybe the devs are using Lineage's boot loader it does a boot.img check or possibly a kernel check and refuses to replace the TWRP recovery image, but if you install from stock HTC it may pass the md5sum test and install, that would be interesting to see.

Right sorry - that was a ramble, but in essence you have a lot more information which you will need to know in the future and htcdev will walk you through the unlock.bin process along with a code that needs to be taken from the device to get the correct file. A word about SD cards. There have been some (quite a few) and microSD card (only up to 128Gb) compatibility which is the M8's greatest feature (IMHO because I get a phone with apps stored on 64Gb+ for £11.99), now I have never had any problems at all, I am currently using a Lexar 133x 64Gb+ card. Another user has reported that a Samsung Evo+ card worked perfectly for either portable or internal (app) storage. I also have a toshiba slow card which during the format it complains but that also worked fine, yet another much more experienced xda member apparently has the same card (hence why it could be a firmware issue) and he can only use it as portable storage and even then files over 300 Mb get corrupted. Anyway look back in this thread for more details, my big experiment with 1.6Gb files was certainly labelled microSD cards and had every method tried and worked every time. I recommend formatting the card "off-device" as something odd like NTFS or apple file system or as anything weird in Linux (android/ TWRP recognises common linux formats) so the device is forced to format it. It will format a GUID partition table as an MBR table so there are no issues if using a Mac to format. My recommendation is format on the device as internal storage and if you need to get the files off you can always use a PC or an OTG cable which is slightly longer than popping out the sdcard but then you have all the storage in the world and lineage keeping it secure for years.

Bugs:

Possible sd card one.

Some people are reporting crashes after heavy loads but that may be a memory leak in an app.

I've have a locked lock screen refusing to respond then catching up, entering every tap it ignore then crashed and reboot.

Built in installer does not auto install updates with TWRP but it can be done manually when you find the file.

There should be an update out today for Lineage.