I used to have one, an hp Jornada 568. Got it for free when I went to Texas, as hp were sponsoring the trip. A large number of them turned out to have a defect where dust would get behind the screen and screw everything up. Bizarrely, mine was unaffeected. I say that because I could be a serious contender for the "Crap Magnet of the Year" award. In the past year and a half alone, I've had a PDA (more on that in a minute), three PS2s (disc read error, played everything except PS2 games, disc read error) and a laptop (screen fault) go belly-up on me. Plus, my Microsoft optical mouse is knackered and my Microsoft ball mouse isn't far behind it.
Anyway, back to the PDA. All was fine until I decided to get a Bluetooth CompactFlash card for it. Unfortunately, since the card was rather wide, it wouldn't fit in the slot properly. My dad had a look at it and tried to force the card in. Unfortunately, he only succeeded in bending the contact pins inside the slot, rendering it useless. I was far from amused.
As compensation, my dad allowed me to swap my now non-expandable PDA for his (he liked mine so much that he got an identical model for himself). That one had the screen defect I mentioned earlier, but it had been sent in to hp for repair so I wasn't bothered. It worked fine for several months when, after I did a soft reset to get out of a crashed program, it just froze. Wouldn't let me do anything. I did a hard reset (erased everything) and started from scratch. Then I had to do another soft reset. Froze again. Done another hard reset, then another soft reset. Froze. Only explanation I can think of is that the screen defect has returned.
Jornadas have been out of production since hp merged with Compaq, and AFAIK they're not supported any more. Mine is now in a box in a drawer awaiting environmentally-friendly disposal.
With regards to wether or not you should invest in one, I tend to agree with what Molendinar wrote.