I have been running JB for a few days now. I haven't noticed any major improvements compared to ICS. There are a few cosmetic changes and the notification drawer adds more functionality. Speed is the same as ICS for me. I never had lag in ICS, though. The Circles Widget is fixed to keep saved settings for notifications.
Some of my apps don't like JB. I may revert back to ICS and go to JB at a later date when more development is done for JB.