I received the JB update (4.1.1, Sense 4+) about two weeks ago now, I live in Canada but I bought an unlocked/unbranded One X from Singapore. Btw my carrier is Rogers. There are notable increases in performance and project butter definitely shows. There are slight revisions to the notification bar and colours, but no need to worry, as these changes are very minor.
The next thing is a 'Power Saver' mode, which brings me to the battery life. My battery life has been destroyed by this update. Even with Power saver on, with moderate usage it dies by 7PM. (Being unplugged at 7AM). This is a major issue. I'm not sure if it's just my One X, but I had terrific battery life on 4.0.4.
Let me know what you think when you receive the update.