I think it might have bricked my phone (maybe?)!
I installed JB using the downloader tool from Samsung's site earlier this week. On Friday, just a few days later, it was dead. I went to check the time that morning, and the phone seemed frozen--nothing on screen, and the blue light was solid. So I reset it. And it kept resetting itself, over and over. So I tried to factory reset. Wouldn't turn on, just the solid blue light.
The phone seemed fine...except for being sluggish and restarting itself and then saying the battery was low (not a battery issue, I checked). The night before it died I planned to do a factory reset, but obviously I didn't make it in time.
I took the phone to a Sprint store and they replaced it for $50. I played dumb and asked if JB had been released for this phone yet; they said it had but that it wasn't good for the phone, that the hardware couldn't support the OS and even Samsung apologized about this problem (?).
So for this new phone I don't think I'll try updating to JB, for fear of doing the same thing again. I plan on maybe getting a Nexus 4 and switching from Sprint to T-Mobile later this year anyways, so I guess I'll get to experience JB then.
I'm curious as to whether this has happened to anyone else!
All that said, I think my favorite thing from the few days I had with JB was the updated pull-down menu. I like how brightness, sync, and settings were added there.