I've been running the
factory image since Saturday (so 4 days). I used downloads 8 and 5 to keep my apps, data, setup intact and then reroot/unlock. Very stable so far. Battery life on par with ICS (19+ hrs for light to moderate use). Google Now is fast! Much faster than the old voice search.
Looks like the gallery WiFi sharing has been disabled (see shared folders for devices on the same network). But it may be a setting I haven't found yet (default off).
Only other issue I have is occasionally the briefing alarm will fail. It will start talking then stop after a second and switch to the specified sound file. But that may be that I switched to the Samsung text to voice a few days before installing JB.
I use Apex launcher, and I have noticed a stutter when panning from screen to screen. Once you've been thru the screens it's fine (cached). But I never noticed this in ICS with Apex.
I have no plans to go back to ICS.