The HTC One A9 was HTC's first device to ship with Android Marshmallow, but despite a relatively clean version of Sense, there's still reason to want to flash a custom ROM. Maybe you want the genuine stock Android look. Maybe you want to cut down on the number of pre-installed apps. Either way, you now have the option to flash CyanogenMod and keep Android 6.0 thanks to the arrival of CM 13 nightly builds.
HTC One A9 (hiaeul) builds are very fresh, as this release marks the first version of CyanogenMod for the device. These only work for the GSM model, not the ones that work with Sprint or Verizon.
CyanogenMod developers aren't stopping at new and shiny devices. The 1st gen Moto E (condor) has also started receiving nightly builds with Android 6.0. For owners of this device, this marks a way to taste Marshmallow for the first time. CyanogenMod 13 has already come to the 3G-only 2nd gen Moto E (otus). The LTE version (surnia) is still waiting.
With whichever device, be on the lookout for bugs. These are developmental releases. Sometimes things are relatively stable. Other times some crucial features don't work. Exercise caution, and don't forget to grab GApps if you want access to Google Play.