There's nothing more flexible and full of potential. You can make arrangements to meet a friend or solve a serious network issue with paper and pen. Most of the apps I used tried to mimic the ease of pen and paper, but none match the flexibility.
For me, the iPhone adds another hurdle with the touch keyboard. Here's the bottleneck: I write faster that I type on a full-sized keyboard, and I do both faster than I use the iPhone keyboard. Some iPhone productivity apps require lots of typing, categorizing, tapping and clicking. Tally up the time spent being "productive" with your iPhone and it's not worth it. Just grab a small notebook.
It only makes sense that I'd love the app from a man who shares my love of paper. Put Things Off for iPhone and iPod touch is a mobile productivity app that I'm happy to use. The initial window (see gallery below) is nice and sparse. To add a new task, click the arrow and type something brief like "Email Tom Re: website." Select a due date and hit Save.
A small "card" appears on the To Do List desktop that you can put in one of four boxes: Inbox (the default), Today (determined by your due date), Put Off and Done. In fact, you might not have to do any clicking at all. The Today box is self-populated, the Inbox is the default and the Done box fills itself as you kill off tasks.
Which leaves the Put Off box. In the app's preferences, you have an option to delay tasks for a certain amount of time. I've opted to put things off for 7 days, just in time for my weekly review. Therefore, items without a specific due date show up weekly, just in time for my weekly review. GTD'ers can think of it as a dynamic @readandrevew or @someday/maybe list.
What I love about Put Things Off is that it comes very close to mimicking paper and pen. I open a note, jot a task and add a due date. Exactly what I do in my notebook. The Put Off box works wonderfully with my weekly reviews and there's no ballet of clicking, categorizing, tagging and whatnot to accompany every entry.
Some will call it "under-featured," but I think it's perfect. Try it out and see if I'm wrong. Put Things Off is available in the App Store now for $2.99US.