If you are the proud owner of the HTC One M9, then you're probably quite happy with your purchase. The device has received a number of criticisms of late, but in fairness, few have been about the performance of the device. Those who do own the One M9 have reported that the device performs extremely well and at the end of the day, that is what matters. That said, the one thing you might not be so keen on (depending on your taste) is HTC's Sense 7.0 UI which comes pre-installed on the One M9 over stock android. If that is the case, then the problem with owning such a new device is that there has been little time for ROM developers to work on an alternative for you.
That said, it does seem that in the last couple of weeks an unofficial version of CyanogenMod 12.1 (CM12) has become available for the One M9. As such, if you are rooted and fancy changing up the OS on your device then there is nothing stopping you giving this ROM a try. It does need to be said, that this is an unofficial version, however, it seems the reports and feedback from users on the ROM are overall positive. Although, there are some issues that you should be aware of. The developer lists that there are a couple of known issues and these include problems with the camera, the FM Radio and the USB OTG. Otherwise, it seems the rest is working well.
In terms of testing, the ROM is noted to have been tested on the Global version as well as the T-Mobile and AT&T variants. If you do fancy giving this one a shot, then remember you do have to be rooted and have a recovery installed like TWRP. Not to mention, you would be wise to make sure you have a backup ready to install in case anything goes wrong. As always with these things, things can go wrong. For those willing to give it a go and to find out more about this CyanogenMod 12.1 ROM for the HTC One M9 click here to head over to the XDA thread.