I also frequently used FoxFi when on ICS and really considered not updating to JB for this one reason. Eventually, I decided to take the plunge so I updated and rooted and use Wifi tether for root so I can still have a hotspot. I appreciate how much faster JB is but i'm having problems with my battery life since the update. While on ICS I would average 28 hours on a charge and JB is almost the same due to SmartActions disabling Data, Wifi, Sync, etc. when the screen is off. When the screen is "Awake" (on), however, my battery power drops rapidly. I've cleared cache and done two FDRs since the update, set brightness to auto & dim, and have nearly all of the same settings/apps that I had with ICS. Unfortunately, when using my phone it discharges at a high rate.
I don't believe the battery is bad since this changed happened immediately. I intend to go through every app at some point to see if something is zapping my power when the screen is on but I don't expect much.
Other than that JB, is wonderful IMO.