When I got my PRS-500 after it first came out, I converted a text copy of the ESV Bible I had into individual rtf files for each book, set up "Old Testament" and "New Testament" in the Reader's "Collections", put the files in the right order, and imported them in. I moved the same collection to my PRS-600 when I bought that last month, and for me it has worked great.
However I note that the same version is available in ePub and said to work on the Sony reader for $15. See http://www.esv.org/blog/2009/06/esv-...n-epub-format/
For me the ESV is most readable and most accurate; I like it better than NIV. And I like having it on my Reader.