I downloaded the update from XDA, unfroze my frozen apps (TiB), used Voodoo root protector and applied the update manually through stock recovery. I did not have to unroot before the update. Don't know if unfreezing was necessary.
At firs the phone ran like dog sh!t but as it turns out it was because all my apps that require root were freaking out. As soon as I restored root and rebooted the phone settled down after a few minutes and has been running great. The UI is so smooth I still can't get over it. There are some other feature changes here and there that I don't really care about (I have no interest in Google Now due to it's need to log my browsing and location history which I turned off) but the UI improvements alone are worth the update. I actually used ADW launcher in ICS and even ADW got better performance too. Also, JB comes with Chrome, which works much better than the stock browser (this was not the case in ICS...it was pretty janky and I didn't use it).
I have read elsewhere that there is currently no root method for JB, but if you use Voodoo root protector to SAVE root on an already rooted ICS Bionic, you can just restore root after the update. I would imagine that if you FXZ (when that becomes possible) you would lose root and not be able to restore it unless a root method is developed. Since you can't FXZ back to ICS once you take this update, if you lose root, you lose root until someone figures out how to do it in JB. That said, if you have an already rooted device Voodoo rootkeeper DOES work and you will NOT lose root if you take the update.
Did I mention how awesome the UI works now?