I had a 3a and found it more convenient to use than the heavy/bulky laptops of the day. My use cases were diary management, expenses, task tracking/todo, and short emails (via the add-on modem/mobile phone cable). The message app also allowed you to send SMS. The batteries seemed to last forever and being a standard size were available everywhere.

I dont know if a "new psion" would sell in sufficient numbers. It's a device I find myself wondering about (along with a chocolate-bar sized version of the iphone (Nokia 6300 size)). I'd buy one of those for sure, for when I'm not working and need a phone, but dont want the full "slab" to carry around with me.