Some people take to PDAs while others never really find the useful. I've been using one for about ten years now, since the early days of the palm pilot. I even thought the "I synch therefore I am" t-shirts were cool. (I'm a super nerd)
I don't have creative uses for mine this is what I do with it.
Address book: A PDA beats a paper address book in a few ways. One biggie is that you have all the room to store info that you want. I can store the names of people's wife and kids, I can have full directions to their house in the notes, etc. Another cool thing is that as they add/change phone numbers, email addresses, etc. it doesn't turn into a scribbled mess. On the downside, I can still write faster than I can enter an address into the form with the keypad. I usually use my PC and synch if I can.
Calendar: My PDA (Treo 650) has become a required brain extension for me. I could never keep track of my meeting schedule without it. Also, sometimes I get into my work and it's easy for a few hours to drift by. It beeps in my pocket, 15, 10, and 5 minutes before my meetings so I don't forget them. Like the address book it's easier to track everything about the meeting than it would be in a paper calendar. I know where it is, the agenda, the dial in phone numbers, who is going, etc.
Notes:I keep lists in here. I have just about every part in my buggy and where it came from in my phone/PDA. This comes in handy when I need a new thermostat/water pump/inner axle seal/tie rod/bearing/whatever and the guy at autozone doesn't want part numbers because their system is based on what car/truck it came out of. When a light bulb blows I make a note on my home depot shopping list. When I want almonds I make a note on my grocery shopping list. I also keep a full maintenance record of all the cars/rigs I maintain so I know when an oil change is coming up, how long it's been since I changed the air filter, etc.
Those are my key uses. Sometimes I play games on it, sometimes I use a program to convert metric to SAE, etc. but those other things are my top three.