The new rendering engine is fantastic, completely alleviates the pentile jaggies you would get with earlier versions of the OS. Everything seems super smooth and sharp with no gross pentile edging.
I upgraded my Yakju 4.1.2 build to Takju 4.2 (keeping all user info and settings) using this tutorial. If you previously moved from a non Yakju/Takju to one of those (unlocking the bootloader in the process) then its just some simple ADB commands and you're good to go. Otherwise you will need to unlock bootloader which clears all user info so probably would be easier to just flash 4.1.2 takju.