LT29i new owner reporting in.
upgraded to JB, looks nice and neat.
improvement on camera settings. i like the superior auto settings, auto-detect every scene for your camera.
and switching rear camera to front camera in just one touch. and yeah, HDR

wifi connected very fast, unlike on ICS, where i had to tap on wifi menu just to see it gets connected.
currently using stock rom, bootloader still no unlock yet.
not yet tested Sony UI launcher for JB, as i prefer with Nova Launcher.
and the keyboard pretty much same as Sony's ICS keyboard.
i have JB keyboard apps for that.
battery wise, it doesnt seem to improve greatly but hey, its a phone. not a notebook. you dont browse internet 24hrs with it, do you ?
but if you go to Settings > Apps > Running, you can see Overheat Control. thats very good.
im satisfied this JB update

recommend all owner to update it

But JB is really really smooth.