I've recently aquired a second-hand SL-5500, and am still exploring it. First impressions are good - it seems to do everything any other PDA does with the added attraction that you can install a terminal app and there you are with a bash prompt !
The keyboard is useful, especially when combined with the on-screen keyboard & handwriting recognition (not as intuitive as the Palm, but you soon get used to it), although if you're coming from a Psion it would be a bit small - you might want to be looking at one of the newer clamshell models.
I think there are ways of syncing to Linux, but it depends on what version of the ROM you have. I'm still looking into that. I spotted some posts on various bulletin boards when looking for info on how big an SD card I can use with it, but forgot to bookmark them. When (if) I find them again I'll post links here.
Looking at the files on my XP box after synching to that, all the important stuff (address book, calender etc) seems to be held as XML, so it should be possible to do something with it.
I've just got a Linksys CF Wireless LAN card, and it's a novel experience to browse the LXF forums from a PDA whilst at the bottom of the garden - I'm most impressed
My next target is to get a large SD card and install ssh & gcc on it so I can really start playing - mind you, I think the keyboard/other input is a bit slow for really serious coding sessions.
I've had a Psion series 3a (used for many years) and a Palm 3C (gave it to the wife after a year - she doesn't really use it either). So far the Zaurus seems to be as user-friendly as the Palm, not quite as good as the Psion for typing on, and far more expandable and connectable than either of them.
Would I recommend it ? I haven't had it long enough to say yet. It's still in the "new toy" stage.
Why not do like I've done & get a second-hand one - have a play, and if you don't like it you should get most of your money back when you sell it again on ebay.