You might be an OmniFocus guru or a Things aficionado. But the Due app for iOS is a valuable addition to any toolbox (TUAW review). It's not for scheduling your life or planning a project. It's for those times when you really don't want to forget something but it's just not going to make it into your task list. Those unplanned imminent tasks.
Due is more of a timer than a scheduler. It's quick in and out to get a reminder set. Not just a reminder, but a nagging reminder that buzzes every minute until you clear the timer.
One side benefit for me is that I can look back at a week and instantly tell how I was feeling. Top three from the end of September was to remember to take a beer out of the freezer.