I have a PRS-505 and have just upgraded :P
PDF reflow is not that big deal of an improvement IMHO, you still could accomplish the same result with pdf2lrf (although it's more of a hassle).
Lacking of support for ePub is not a big dealbreaker either at this moment, though things might change in the future as more publishers support that format.
The general (slight) performance improvement is nice though ... but it's optional. So don't worry too much, you PRS-500 guys are not missing a lot by not getting the firmware update.