CyanogenMod has been working on CM11 since the end of last year. The first CM11 code was pushed to GitHub on November 6 and the first nightly releases of the KitKat based ROM started rolling out on December 5. We're a month and a half into 2014 now, and the Cyanogen Inc team has just released a new M3 build for select devices.
These M3 builds are designed to showcase new features and such, but are less buggy and are generally more stable than CM nightly builds. If you're the type of person who is a little uncofmrotable running bleeding edge nightly firmware on your device, the M releases might be a little more your style. M builds are released about once a month, and are stepping-stones to the stable release candidate builds that the CM team drops every few months. I usually don't recommend nightly builds of any ROM to the novice or casual ROM flasher but these M builds are almost always rock solid. CM11 M3 is no different. It's more than stable enough to be a daily driver for you, but it may be lacking some of the spit and polish that the next RC CM11 build will have.
This newest M3 release is not available for every device yet, but it's out for the Nexus 5, Nexus 4, Galaxy Nexus, ASUS Google Nexus 7 (2013), LG G2, and the Galaxy S4, among others. More smartphones and tablets will be added in the coming days and weeks. If your device is not supported yet, don't worry, you won't have long to wait. Head on over to the CM download page and see if you're device build is up yet.
As with any custom ROM or tinkering you undertake with your devices, keep in mind that you move forward at your own risk. Any issues you run into or warranties you void are your own responsibility. The Android community is great, though. If you hit a snag or find a bug, do a little searching on Rootzwiki or XDA Forums. You'll find the assistance you need. And always remember to have fun! If you're not having fun, you're not doing it right.