Now that my wife and I both have iPhones running iOS 5, the single most useful feature for us is Reminders. I’ve used to do lists on and off for years, but never found anything that was always accessible and easy to use. With iOS 5 and iCloud, I have access to my reminders on my desktop, iPhone and iPad. Furthermore, with Siri on my iPhone 4S, I can add reminders when I’m running or when I’m driving.
To top off my personal use of reminders, with iCloud, I can share reminder lists with my wife. I’ve setup a grocery list that we can both see. The other day I added an item to the list while I was in Portland and when I arrived home later that day, my wife had been to the store and purchased everything on the list, including what I had added!
There are lots of other useful iOS 5 features, but so far, this feature could be something that helps restore my sanity. Yes, I know that other apps can do this, but this is simple to use and just works.