I've owned quite a few mobile devices, and none have been able to do what the iPhone has done for me. The iPhone has helped me with school, work, blogging, social events, and personal development. Here's how:
With School: Last semester I took a psychology course that was partially online based. Several days throughout the week, we were required to communicate on the message boards and take exams. Having the busy schedule that I did, I wasnt always in front of a computer. I've taken exams at the gym, posted replies while driving in a car, and communicated with the professor from the movie theatre. Some things just can't be done so well on other devices.
With work: When I used to work retail, there were times where I had to swap schedules, let HR know about my delays, and receive special notes from the team members regarding updates. In terms of my design clients, I can't exactly use Photoshop on the road, but helping me stay in touch with them has been a tremendous benefit. More after the jump...
With Blogging & Microblogging: I usually write down a lot of my ideas using the Notes application for later use. I'm on twitter all the time (pockettweets) using my iPhone. I've posted comments, replied to emails, and complained about all sorts of things on twitter.
With General Things: From checking the weather to seeing the latest product reviews, the iPhone has come in handy in the strangest of times. Checking dictionary.com and browsing Wikipedia for the sake of proving points and winning arguments has never been more fun.
With Personal Development: Before I got the iPhone, I never had a quick way of keeping dates and events organized, other than forceful memorization. Calendar reminders, alarms, and notes have kept me on par with the most important things. When I ride the subway, my thought process usually kicks off. Having the iPhone with me is a huge plus since I no longer forget those "great ideas."
So, on top of all that, I think it's worth mentioning that this post was written on an iPhone. Just goes to show that a smart phone is the best choice as a mobile device for someone with a busy schedule and lots to do.