I have used a PDA since grad school and used it a lot in first and second year med school (particularly for keeping my schedule up to date, tracking my grades via a program called 4.0 student, taking notes in class with fold out keyboard, looking up countless words in Stedman's for PDA, looking up countless drugs in free epocrates, looking at netter drawings via netter software, studying material ANYWHERE with flashcards from a program called SuperMemo, reading my text books where I didn't want to carry them (palm version books - my palm should weigh in close to 100 pounds based on the amount of books I have on it)).
Keep in mind if you get one now you will need a new one most likely before you graduate (to keep up with the tech). If you are going to get one, the lowest model you should get is a Tungsten E2. Your next decision (after deciding whether or not to get a PDA) is how important wifi vs. phone option is to you. I do not think the treos have wifi ability but am not sure, but they do have phone internet connectivity which of course costs extra. If you're not going to use this thing to it's full capability then it may not be worth it for you. However, if your school is a case/problem based type I would think having all your sources within clickable reach would be extremely helpful as opposed to lugging books around all the time and I never once used a notebook to write notes throughout all of med school (if I wrote anything by hand it was on handouts provided).
Finally, do you really NEED it? Let me put it this way, you don't absolutley NEED any one particular thing in med school. You will hear people tell you all sorts of things. What's most important is what works for YOU, and for ME a PDA has been incredibly helpful.
-J