Having just got a PDA for my wife (Palm T|X), I see how we are different. She has become, in under a week, a PDA junkie, while I can't imagine moving to a PDA from my laptop (though I acknowledge a PDA's benefits). Here's my take:
1) PDA's can play media well. I found the video quality on our Palm to be excellent - I wouldn't have a problem watching a 2 hour movie on it in landscape mode. Obviously, if you are used to 17" widescreen glory, it takes a bit of getting used to, but I find that people in general can adjust pretty well to changes in video screen size. Our Palm is about 20" smaller than a standard 4x6 picture. The screen real estate looks huge for the form factor.
2) For PIM stuff, the PDA can't be beat. It is designed for this. Tasks, calendars, contacts...my wife (a list person) is in heaven.
3) The PDA turns on/off almost instantaneously, and when you do turn it back on, you are back exactly where you left off. Hibernation on steroids.
4) Great battery life. With non-volatile memory, you won't lose anything if your battery happens to die on you.
5) Synching works well, and with WiFi or Bluetooth, you don't get saddled with a bunch of cords.
1) PDA's are extremely portable, but not having a full keyboard sucks. You can get an external keyboard, but you lose your portability (the keyboards are small and foldable, but you need a flat surface. If you're going to work on a flat surface, get a laptop). I also have a Blackberry that has a full keyboard built in, and that's bearable. So I would highly recommend a PDA with a built-in keyboard if you go this route.
2) PDA's can surf the web, but the screen size is limiting. We do simple things like check the weather forecast or see the news headlines. And even for this, unless you find some PDA-friendly sites, it can be a bit of a pain to browse. The screen resolution on our Palm is 320x480 (I think) which is quite large by PDA standards, but still not comfortable for general browsing.
3) PDAs can get and manage email, but you may not have full support of all attachment types.
4) Software support. There's a fair amount of software out there, but there will always be more on a laptop. This, to me, is the kicker. Figure out as best as you can what you need to do, and take a look at what PDA software offerings there are.
I say, if you're like my wife who wants to manage tasks, contacts, send/receive email and browse the occasional website, get a PDA. The form factor, battery life, and at-your-fingertips availability can't be beat.
If you're looking to do anything more, get a laptop.