I can't help but see the parallel between the success of the iPhone due to its sexy image and a massive deveoper community producing hundreds/thousands of apps for all sorts of weird and wonderful uses, and the success of the Psion Series 3.
The Series 5 and 7 never really matched their success, given that PDAs had gone mainstream by then, and I don't think it's coincidental that the same kind of developer community didn't emerge after the Series 5 launch.
There were certainly some ver nice and powerful apps, but not over 3000 programs covering every need like the Series 3.
Perhaps there's even a parallel between that and the success of Firefox and its extensions? Regardless of what people actually do, they just like buying into something that seems so versatile?
But perhaps any new Psion today would have little in the way of physical advantages. I can't see any 'proper' device still running off AA batteries, and the screens on any device are miles better than even the original Series 3a. Integrated connectivity ports are standard, as is some kind of wireless link, and even small keyboards have come a long way in the last ten years, though the Series 5 is still lovely to use, and the Series 7's should have been licenced to laptop manufacturers in the 90s because even by today's standards it's magnificent.
Shame on Nokia & co for not licencing or buying Psion's patents to make their hardware better years ago. No one's benefitted from them ever since, not even Psion themselves (when was the last time Texlogik released a device using a Series 5 keyboard?).