The Omnigroup has created the best implementation of productivity and GTD (Getting Things Done) software for Mac and iOS with OmniFocus. I have been using this software for the past 6 months and have had the chance to try it out on the Mac, iPhone 4S and new iPad. The Omni Group folks are really responsive to any support questions as well as trying to integrate new features of the iOS 5 operating system.
What I really love about this software is that it keeps each program I have on every device synced and up to date with every change using their OmniSync server. I also love using Apple's Siri software on the iPhone 4S to create reminders, sync up to iCloud and then have OmniFocus pull the reminders from iCloud down into my OmniFocus inbox complete with due dates or locations reminders if they are stored in the respective reminder fields.
It was a little tricky trying to get all of the syncing set up properly and required a few emails back and forth with the support staff, but they were really determined to assist me and even responded to me over the weekend! Talk about dedication...
All in all, while it may take quite a while to implement the GTD methodology or your own productivity solution, I feel that the OmniFocus software is the best out there for Apple systems. It would be nice if there were some web-based interface for the OmniSync server to be able to use in a pinch, but all in all this software is well worth the money and Omni's dedication to development of the software keeps them at the head of the pack. The key to this software is that they make it available on all of your devices so that you always have it with you.
As a parent and homeowner, having a centralized location to store all of my to-do lists and everything that I am working on is key. OmniFocus is my trusted system to be able to get things done.
-- David