Windows Pocket PC or WinCE or whatever they're calling it these days looks nice and works well, so long as you never touch anything. Proper VGA support is spotty, rotating the screen can cause issues, and the software is about as flexible as your average brick. Not to mention the horrible lack of a keyboard!
OTOH, the built in connectivity is nice, there's lots of software out there as long as you don't mind paying for it, and it synchronizes well with windows and outlook. (So long as Activesync, which can be a very annoying piece of software, even in its latest versions, isn't feeling cranky.)
If you want software that *just works* for PIM, go for Palm. Palm's networking software, like web-browsing, sucks pretty harshly though.
In short:
Palm's hardware is Okay, Pim is great, web sucks, poor flexibility.
MS's hardware is great, Pim is okay, web is okay, okay flexibility.
Z's hardware is great for different reasons than MS's, Pim sucks, web is great, great flexibility.
That's my take on things. I'm a geek, so I completely abandoned over 5 years of using a Palm the instant I got my 860. I've tried Microsoft based PDA's repeatedly over the years and I've never once been satisfied with the stability or usability.