You can (and I have) fetch the new Launcher from AOSP, compile it and run it very easily as a separate application. The Google Experience Launcher is a closed source Google app based on this which integrates Google Now.
https://android.googlesource.com/platform/packages/apps/Laun...
I now know which app I'm ditching once I get a 4.4 device.
I'm a huge fan of Google Now so hopefully I can continue to use it if I decide to switch to a custom launcher.
I now know which app I'm ditching once I get a 4.4 device.