Please note that this is a leaked pre-release ROM, which doesn't seem to be polished just yet. This firmware is labeled "I9300XXDLG4", and has been built on July 31.
Some of the visible changes brought by the Jelly Bean ROM (apart from the under-the-hood goodies coming with this version of the OS) include a new clock in the notification bar, Google Now and some new live wallpapers. However, issues aren't nonexistent as well. For example, SamMobile reports that it is not allowed to install some apps, due to a missing file "smdl2tmp1.asec".
If you aren't too impatient, it'll probably be best for you to wait until the end of this month, when Samsung is expected to roll out the final Jelly Bean firmware for the Galaxy S III. However, if you won't be able to sleep until then, it might not be a bad idea to try this out, just be sure to be extra careful when installing it, and don't forget to back up all important data first.
Head over to the source to find instructions on how to get the leaked JB software on your Galaxy S III.
source: SamMobile via SlashGear