First round of nightlies ready for flashing
If you're ready to rid yourself of HTC Sense 6 on your M8, you now have a chance to give CyanogenMod a spin. The free and open-source version of CM has hit nightly status for the metal-clad HTC flagship, and the builds are currently available at the CM downloads site.
Be warned, that as nightly versions there will be issues to work out. You are doing the community testing for the builds, and should be prepared to submit bug reports and do your part to make things better. Don't worry, though, usually these are pretty complete and alpha/beta testing is often a lot of fun.