I'm using the new 5mx PRO, which in many respects is just a faster series 5 with a marginally better screen. If you're looking for a desktop you can fit in your jacket pocket, the Psion 5mx is a good choice - but be aware that hardware problems are likely somewhere down the road. I've resisted getting any PDA for a long time. I definitely needed one for organizing myself, but I feared that one more programmable toy would completely consume the little non-computer time I have in my life. But then I spent a couple of days working with someone who had a Psion 5mx.
Writing is slow, and handwriting recognition is dumb.
The Psion keyboard is remarkably usable considering the size. PsiWin 2.3 synchronizes Agenda with Outlook without any problems whatsoever. CF disks are usable in many digital cameras. It's a real computer and it runs Java. I'm a programmer, and I love the idea of being able to hack Java on this thing.