Sorry to stick to this issue, but I disagree quite strongly on this. Let me start by saying that I love the Psion devices. The Psion 5 still does some things better than nearly any device out there in the market. I think it was a wildly successful device in many terms: engineering, design etc. But I cannot really buy "wildly successful" in terms of popularity and sales. Nor to any other non-connected PDA device. Zaurus was discontinued some years ago. I don't believe it's a question of "not marketing". Truly great devices and services gain popularity with "zero marketing", and vice versa, no amount of marketing can help to really sell a non-useful device or service.
Quoting numbers from: http://www.ericlindsay.com/epoc/mhist.htm Be it 25000 Psions a month or 163000 devices year or whatever, that is nowhere near even moderately successful. By successful I mean mass market appeal. A device that the majority of people would seem worthwhile to buy. PDA's focusing on personal information management have never reached this target. Mobile phones are wildly successful.
I do agree with this thing, however. I use PIM functions on my E61 phone, since it is my primary device. I use the PC suite to automatically sync with my Outlook calendar. Even if my tablet would have PIM functions, I simply wouldn't use them. Writing down the entries wouldn't be as comfortable or as fast as with my phone. I don't carry the tablet in my pocket all the time so that I could check my calendar or notes.