I tried using regular planners, notepads, etc., but I just wasn’t motivated to keep using it, keep it updated, etc. I love gadgets, though - so I bring my PDA everywhere and use it. I have a horrible memory, so when a thought strikes me of something I should remember (movie to rent, groceries to buy, appointment to make), I write it down. With the sheer amount of data I have saved in there - everything from vaccination records (which I needed for work at the hospital I’m currently at), references, book lists, and so forth, I’d need a pretty big planner to keep it all in. (And I know that for certain, I’ve tried.) My PDA is compact and light, and I can back up the data at home and at work.
I play games on it during my train commutes, or look over my shopping list in the store.
I have a scientific calculator and a units conversion application for when I see patients. My boss has a prescription drug reference guide loaded onto hers.
You don’t need a PDA. I do, though; it makes my life a lot easier.