Stephen
I have the earlier version (PRS500), and can highly recommend it. The reflective screen, size, and weight really do make this feel like reading a book. As someone with 5,000+ books and a family that won't let me put up any more bookshelves, this is a godsend.
You're not tied to the Sony store - there's limited support for PDFs, and you can convert any text format into the internal format; lots of goodies available from Baen and Fictionwise, for example.
My one criticism is the screen resolution, which could do with being a bit higher.