I agree with the "2 devices - 2 uses" mentality.
I have both a Kindle2 and an N810. I use them for two totally different uses, with very little overlap. I use my kindle for reading ebooks during the day at my kid's activities, because it is simply easier to read in the daylight. I use it to listen to music while I read my books. I also use the built in web browser to quickly check emails and activitiy schedules/updates.

I use my N810 as a PDA, to hook up to m car stereo for music on trips, as my GPS, and as a ultra portable computer.

To expand even further, I use my netbook for surfing while watching TV and carrying to conferences and the like so that I always have Internet.

I use my desktop when I need real computing power and multitasking.

I use my Blackberry to keep me informed of my WORK schedules and email.

So the moral of the story is that each device has its own use and NO, I do not carry all of them with me all of the time, but even when I travel, all except the desktop fit in my sachel and away I go......