FlyMe OS is the one of Android custom ROM that developed by Meizu and now that custom ROM has been update which uses Android 4.4 KitKat operating system, since the KitKat is the newer Android version for now.
The Meizu MX3 and MX2 user is now ready to get FlyMe OS that uses Android 4.4 based OS, unfortunately, the new FlyMe OS version is only available for those Meizu series for a while now. The main reason is the Meizu MX3 is the one of popular smartphone from Chinese manufacturer.
The new FlyMe OS that uses Android 4.4 based OS is running more smoother than older version, so the user can install the stable version with a lots of new features.
Back on few months ago, Meizu founder, J. Woong has been said that FlyMe OS is also ready to available in other device, while for now there is no information about the release date but seems Meizu is following the Xiaomi that also launch the MIUI interface for other device rather than Xiaomi devices. [source]