Back when I worked at the Ensign magazine, they used to buy us the planner of our choice, and I opted for the wallet-sized Day-Timer kind. Since I left the Ensign, I've continued to pay about $25 a year to refill the planner, and my wife got into the habit of using it for her planner as well (she often calls me at work to have me add a calendar item or ask what's on the calendar).
I knew the iPhone had a calendar feature, but I've never really liked PDAs and stuff like that, because they're more cumbersome than plain old pen and paper. But the other day at lunch my boss was fooling around with my iPhone and teased me about not using the calendar function, so I decided to open it up and experiment with it. (To bug him, I sometimes call it my "iPod phone.")
By the end of the day, I had input my whole planner and also my phone/address directory into the iPhone, and now my wallet is much thinner.
The iPhone calendar works great, but it's not quite as fast and direct as pen and paper—whenever I need to input a new item, I feel a slight drag thinking of all those buttons I've gotta push. But I can set up recurring items and set alarms to remind me of things, and I do like having a slimmer wallet—before, I often had to take my wallet out when I sat down, because its thickness bothered my buttock. (Please do not quote the last part of that sentence out of context.)
I'm sure I could be using technology even better to sync up my work calendar with my iPhone and stuff, but for now I'm just inputting everything directly into my iPhone. One downside is that I'll no longer have a physical diary to keep on hand as a historical record, but oh well. And the more I rely on my iPhone, the more I hope I don't ever lose it or break it!