I've never rooted my phone and I was on ICS prior to this upgrade. I'm happy to report that my phone upgrade to JB 4.1.2 went well earlier today, and I have no issues to report. I did loose my FoxFi tether ability as other people mentioned but over all, I'm happy I upgraded. The O/S runs better than ICS in my opinion and it feels smoother.
I just wanted to let others know that not everyone is having problems. I've played with all my apps, and everything seems to be working. I did down load the "All -In-One" app and I killed any processes that I didn't want running. The battery seems to be lasting longer and it's not getting as hot. I'll report back in a few days and update my post.
Edit 3/28/2013 On the physical keyboard, I've verified that I cannot do any "shift" functions so all the symbols in "yellow" are not working on my phone.
Edit 4/1/2013 - In order to get my key board to work, I had to do a factory reset. I no longer have any issues.