You can equally well use PDFLRF on other platforms, so let's not get tied down to the Sony too quickly.
You should also consider the Bookeen CyBook, since it has excellent dictionary lookup facilities - something which the Sony lacks, and you mention as being important to you.
Jon will doubtless chime in here and start his usual moan about how the CyBook is useless for displaying pictures. He's exagerating - most pictures are perfectly acceptable
.