Background- first year engineering student.
I don't have one mostly because I find that I'm taking my laptop with me to meetings or other times when I would need the calendaring, contact list, or task list features. (I suppose that might make me a nerd). This is partly because it's just useful to have for taking notes, or a quick email followup to someone not at the meeting (actually, for getting people not at student government meetings to volunteer for tasks, IM is great), and partly b/c if the meeting's boring, I like to have something else to do (email, read news).
For the times when I'm not carrying my laptop, it isn't that bad to just go and write something down or put a phone number in my cell phone.
What would make me get a PDA is a decently priced ($450) PDA Phone with reasonable data input and battery life, as well as 802.11b support. It's getting close- there's the Smart phone (Kyocera, I think...), Audiovox Thera, and Tmobile Pocket PC phone edition, Handspring Treo, but there still isn't the "right" one. I need something that will replace my phone (with the same, or close to the same, talk and standby time and not too much bigger) and would mean that I don't have to carry my laptop around quite as much.
I'm hoping they get there within a year, 18 months is probably more reasonable.