Hi, I used a Palm for the last couple of years of medical school, and have been using one while working as a doctor. I have found them to be very useful, first of all as a diary, and also for storing textbooks and notes on.
I'm currently using a Treo 650 phone/PDA which I bought secondhand, and find it to be very good. The battery life is excellent, the screen is nice and big, and it doesn't take up much space in your pocket. I would keep the UK drugs formulary (BNF) on it, and have used epocrates also. In addition, I have the Oxford Handbook of Clinical Medicine, medcalc and dxsaurus for medical things.
If you don't need the phone part, I previously had a T5, which was great, a pleasure to use. The latest version, the TX seems good also, and the new Treo 680 looks like a good PDA.
I found the built-in calendar to be very good, and I don't like diaries!
Andrew