I've used a Palm PDA without interruption since the first one was introduced in 1996. I was working for U.S. Robotics which owned Palm at the time and the employee pricing helped me decide to take the plunge into PDA life. After all these years, I've come to rely heavily upon a few key PDA applications (in addition to the standard PDA applications).
I use SplashID to securely store the multitude of passwords I need to remember both at work and at home. Without it, I'd have to resort to using weak passwords in order to stand a chance of remembering them all which compromises security.
I use SplashMoney to record credit card transactions while I'm away from my computer. This ensures I stay within budget and helps guarantee that I recognize any erroneous charges which might pop up.
JFile is invaluable for storing databases I design myself. I use this to keep track of all manner of data such as books I've got and those I'm interested in reading. Before I did this, I occasionally bought multiple copies of a book.
SlovoEd is a portable dictionary which allows me to look up words I don't recognize when reading without a print dictionary handy. My Centro takes up a lot less space on my nightstand than a conventional dictionary.
Adobe Reader for Palm allows me to read PDF documents on my PDA. This is handy to ready books in non-traditional settings. It's nice always having a book handy to read for those occasions when unexectedly left with extra time to kill.
A couple months ago, the time seemed ideal to upgrade my phone and PDA. My wife's phone was acting up and my stepdaughter wanted a cheap PDA. So it made sense to get a device which fulfilled both those functions for me, freeing up my phone and PDA for them. This also had the added benefit of allowing me to pare down the devices I carried from two to just a single gadget.
The Palm Centro is smaller than I expected but keyboard surprisingly useable. The software upgrades work to make the smaller sized device more intuitive to use than older Palm devices. Is it perfect? No, but it does seem a better compromise device than the other affordable multi-use devices I've seen.
If you're interested in an affordable combination mobile phone and PDA device, check out this review from Engadget.