Okay, I've flashed the leaked XXUFMB3 firmware.

Differences from Jellybean 4.1:
  1. Lockscreen Widgets: Yes, they're present! You can unlock straight to camera!
  2. Notification Drawer: It doesn't look like the fancy 4.2 notification drawer. The secondary notification menu icon does not give us 4.2-like quick settings. Instead, it just expands the icons we already have. Which means that the quick settings icons (Wifi, GPS, Bluetooth, etc.) do not scroll! Also, the whole thing now has a blue background. The date and time are differently positioned.
  3. Gallery: The title bar when viewing pictures is transparent! Excellent!
  4. Extra Applications: New Message shortcut in the app drawer, Jewish calender app, OfficeSuite, and Snap n Go.
  5. Settings: Here are the new things in settings:
    • My Sound: Under the sound settings, we have My Sound. This used to be in the Phone settings before.
    • Daydream: Yes, we get Daydream!
    • Location Services: We have a new toggle for Access to my location, and a My Places option.
    • Lockscreen: The settings have been changed around and you can access your lockscreen widgets from here.
    • Language and Input: Driving Mode has been added here.
    • Cloud: The Cloud option makes its return.
    • Voice Command: A completely new option which helps you control the alarm, camera, music, radio, and calls. This used to be a part of S-Voice. You can tell that the ROM is incomplete because this doesn't have an icon.
    • About Device: The Status now tells you if your device is official or not.
  6. RAM: Total available RAM is now 828MB.
  7. Screen Timeout: The home button to wake-up is no longer tied to S-Voice!
  8. One-finger expandable notifications: Yes, they're present now! To collapse, you have to swipe down a bit before swiping up.
  9. App Permissions: The app permissions are cleaner and have the 4.2 style.
  10. Smart Pause and Smart Scroll: These can be accessed by long pressing Smart Stay in the extended notification menu.
  11. Minimize Animation: There's a new 4.2 Jellybean animation when you long press the home button whilst using an app. It looks really good!
  12. Default App Selection Skip: When asked to open a file with a certain application, you can now double tap on any application to quickly open it instead of making it a default or opening it just once.

Issues:
  1. Not smooth: It's not smooth. Not by a long shot. It stutters at times and there's frame tearing when you scroll really fast. This experience may vary if you've wiped!
  2. Gallery: The album names show up in bold when viewing an album in landscape mode. It looks ugly.
  3. Contacts/ Phone: Yes, the lag is still there whilst opening them for the first time.
  4. Motion: Double tap to top doesn't seem to be working.
  5. Touchwiz: It still redraws with heavy usage and the shadows redraw sometimes, too.
  6. Multi-tasking: Multi-tasking with games is still broken. Eventually, one of the games will restart if you have more than one game open.
  7. S-Planner: S-Planner force closes on week- and day-view.
  8. Home Screen Shadow: There is a dark shadow at the top of the home screens in Touchwiz.

Final Impressions:
This is just a leaked firmware so expect more issues to pop up! If you really want to test it out, you can flash it. Otherwise, it's nothing to die for. It isn't as impressive as I thought it would be but it's a good step in the right direction!
Also, a wipe is highly recommended otherwise you will face a lot of issues (and lag).
Android devices owned: Samsung Galaxy S GT-I9000, 8GB, Samsung Galaxy S3 GT-I9300, 16GB (Pebble Blue), Nexus 7 (2013), 32GB, Nexus 5, 32 GB