I love the 505 too. I've had it for almost a year now and never regretted the purchase even at a 300Euro price. It was just something I had to have.
I'm reading more now, too. Only thing I'm missing is panning when zoomed or that pdfs cannot be zoomed the same way as images.
PRS+ adds functions the reader doesn't have with the original firmware. My reader provides now (more or less) all functions I would have loved to see on it right from Sony.
This is the direct link to the Usermanual wiki. This should give you a good overview about the new functions.
http://code.google.com/p/prs-plus/wiki/UserManual