There's nothing really wrong with MJ5. It depends on the carrier. AT&T and Verizon were already locked down tight at MI9, the others are still fairly open at MJ5. My wife's came with MI9. She let it update. Mine came with MJ5. About the only difference she noticed with the update was the color of the shift arrow in the Samsung keyboard. (Samsung doesn't release updates for unimportant things, you see.)
As far as the next update, to KitKat? I won't be accepting that until it's been out for at least 2 or 3 months, and I haven't heard of functions ruined of phones bricked. The S3 4.3 update is still too fresh in the minds of some of us.
I don't know if a rooted phone even WILL update if you accept it, but the easiest thing to do is wait for an update notification, then long-press the notification. You'll get an "App info" button. Press that and you'll be at the app that notified you of the update, Clear its cache, then clear its data. That should stop it for a while.
