I've used one for the past 5-6 years.  I use it for both business and personal information...I keep everything in it.

I would stay away from the PalmOS, go with the Windows version.  It will connect and sync right out of the box, and while the PalmOS will work just fine once you get it set up, I have found them to be a little more trouble.  Also, PalmOS seems to be going under.  It was spun off from the parent company and isn't doing too well.
Windows PocketPC also has a version of MSOffice on it so you can work on documents if you like.  It also has media player and all that.
If you have the $$, you can combine your pda with your cellphone and have one less thing to carry around.  I am waiting for the '06 models to come out so I can get a good deal on an '05.
If you have access, the newer ones have WiFi compatibility, so you can connect directly to your network or internet.

Just some background on me, I do IT for a fairly large chemical company right across the PA/DE line, so I get to see alot of these things (well, usually when they are not working right)

I'm probably not much help in regards to new PDAs. I have been using the same Palm IIIxe for over five years.