While manufacturers work diligently to get Android 6.0 Marshmallow into the hands of their users, this is merely one way to get a hold of the latest Android software version from Google. It's also the official method, but there are other more speedy routes of obtaining Android 6.0, and one such method is through the means of unofficial updates, or custom ROMs. CyanogenMod, probably the world's most popular custom ROM for Android devices has just released a new batch of nightlies for CyanogenMod 13 which is based on Android 6.0 Marshmallow. The devices which the nightlies were released for are the Moto E 1st Gen from Motorola, and the GSM model of the more recently released HTC One A9.
Although the HTC One A9 already had Marshmallow onboard at the time of launch, not all users will care for the stock software experience provided to them by HTC and will look for a different user experience. Alternatively, other users may actually quite like the Sense UI which shipped with the HTC One A9, but still have a thing for rooted devices with a custom ROM onboard. For all of those users regardless of the reason, the arrival of CM13 nightlies means that they can still keep the capabilities and features of Android 6.0 but with a more customizable firmware. It's worth mentioning as well that CM13 is only available for GSM models of the HTC One A9.
As for Motorola Moto E 1st Gen owners, Marshmallow wasn't in the cards so CyanogenMod is helping those users update to the latest version of Android software where it wasn't an option officially through updates from Motorola themselves. As with all custom firmware, especially nightly builds which are intended to be more early stage and experimental, there are likely going to be some bugs and alongside them, improvements that will need to be made to fix any and all issues which appear. Having said that, any users with either of these devices should be aware of the risks that are involved with flashing custom firmware, and even if the installation process goes smoothly it's advantageous to be prepared for certain functions not to work. If there are any issues, the good thing about nightly builds is that the updates are usually coming through to devices on a daily basis, or at the very least every few days, which means any issues that are present usually aren't around for long.