Hello,
Regarding Psion connectivity, there is a title available at Psion's online store calles "MacConnect". It offers backup utility only (no conversion). It retails for about $50 USD. It also appears on ebay pretty frequently.
For conversion, there are two freeware titles available that work in conjunction with MacConnect. They convert text files only between the mac and psion. I don't know what they are called, but one is from Atelier (http://www.ateliersoftware.com/epoc/mac5.html), and the other from Enfour (http://pda.tucows.com/epoc/preview/25700.html). Like I said, both are free downloads.
There is also an agenda interface from a company in the UK called Propedia (http://home.clara.net/mugen/EO.HTM).
But the GOOD news is that Psion has re-committed to the Mac platform, and is expected to release a much improved and more comprehensive Macintosh interface sometime soon (http://www.theregister.co.uk/content/1/13052.html). Rumor is that Psion will make a new product announcement on March 1. Perhaps this new software will be part of the announcement.
I've tried Palm and Visor, and they are ok, but toys compared to the Psion. Unless Apple gets back into the handheld market (and it TRUELY should), I think that Psion is still the way to go. I just hope that the new Psion software for the Mac is top notch.
Good luck with the series 5 Psion. Hope that this helps.
-Briguy