I used ToDo on the iPhone with Toodledo on the web for years, and I was quite happy with it. However when I tried to update ToDo, as it was increasingly buggy, to ToDo Cloud, the install failed and I started looking around. And I am glad I did. I looked at many reviews and decided to give Things 3 a try.
The tutorial is well done, and in 15 minutes I was fully up to speed. I then reentered about 100 tasks, although I could have imported them. I have to state that the interface is a joy to work with, and the final result was amazing.
Read the reviews for product details, but this app defines what is great about iOS. Clean, intuitive and fun. The integration with the calendar, in particular, is brilliant.
After getting it set up on the iPhone, I purchased the iPad app. Perfect integration and interface!
My only quibbles... When I set an Inbox item to Today, and then I do not complete the task and clear the date, the task ends up in the Anytime folder. I think it should go back to the Inbox. This is by design, and I have a workaround, but it is mildly annoying. Also, I would like to not display the Anytime folder, as I do not need it, but this is not customizable. I could use the real estate for another area or project. But these are minor details. This is one of the best apps for iOS. Period.