You can't build an immediate response device on a multitasking general purpose computer.
You can get all of the functions in there, with some degree of reliability,
but it can never be any where near as efficient to use as a dedicated device.
Would anyone propose replacing their cellphone with a WiMax laptop running VOIP software?
Somebody's calling, let me get this out and boot it up.

My Palm has long been an extension of my brain.
I can retrieve the information I need from it with one hand, in seconds,
and doing so doesn't interrupt the flow of my current thought.
The Treo is small enough that it's always in my pocket.
Battery life is good enough that I don't even think about it.
Add to that it's function as an ultra-reliable alarm for when things need to happen,
and I would find it very difficult to function without it.

People complain that Palm OS hasn't been updated in forever.
When was the last time pencil and paper were updated?
It's a mature technology. It does what it does, extremely well.
I actually wish they would un-update some parts.
Grafitti 2 worked really well for me on the Tungsten C.
It's broken on the Treo 650 and, I believe, all Treos since.

My N810 makes a superb companion to my Treo 650,
but, if I'm only bringing one, there's no competition.

Daytimer -> Palm IIIx -> Tungsten T -> Tungsten C -> Nokia N810 -> Treo 650 + N810