I bought prs-500 in Jan 2007 just for an occasional trip and summer holidays, so I don't have to bring with me 2 hard back editions...In no time I fell in love with it. I never expected it to happen! Bought prs-505 in December 2007. Love it even more. I don't care about features but the screen is so much better: more contrast, brighter and no ghosting at all. The reader goes with me to work everyday.
I am not going to buy iRex at least in the near future, or whatever comes out from other makers in Spring 2009 only because I hope Sony is going to make something similar 9.7" size and I trust them more.
P.S. Sony! Don't let me down!