I've been playing a bit with the Nexus5 , and coming from a Galaxy Nexus this thing is blazing fast! Google Now starts instantly, apps work great and haven't seen any lag so far, not even when installing apps.
On the other hand, as I was expecting, I'm missing things present in Cyanogenmod , such as custom profiles (default, home, work, etc.), customizing the notification LED straight from the OS and other small things such as the advanced power menu with reboot, screenshot and so on.
Oh, and right now the Xposed Framework isn't working on KitKat , so if you were using it and Greenify to both hibernate apps but still get push notifications for the ones using GCM, you're out of luck.
Going to see how the battery holds, but I'm sure that once Xposed gets going things will be even better.