See this thread:-
and this Google Group:-
Basically it gives things like sorting into folders and a clock in the corner that does not come natively. I waited until the PRS505 was out of warranty before trying it and certainly the clock thing is handy and something I miss on my Kindle.
There are some good screenshots on the thread.
I was at a school prizegiving last night for my grandaughter so haven't managed to get the photos up yet, but will do them as soon as possible.