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I consider myself tied to Outlook too because I like it for email, calendar and contacts. I used the GTD Add-in for about 6 years and, before that, had it configured for GTD according to the Davidco whitepaper (I've been practicing GTD for 10+ years.)
When I got an iPad I saw an opportunity to move around more freely without my laptop. My Outlook calendar, contacts, and notes sync with it beautifully but not the tasks. I had heard SO MUCH about Omnifocus that I decided to take a look at the app. I had a bunch of questions, so I emailed customer service and told them what I planned to do (you can also call them and discuss it.)
Omnifocus for the iPad is strictly a list/project manager so it closes the loop in my system. If I want to turn an email into a task from my computer, I send it to a special Omnifocus address. I then open that email on my iPad, tap it, and it opens as a task in Omnifocus! It takes just seconds. So I keep my computer to my left while I'm working and my iPad is in the center of my main desk--ready to go--at all times. I sync it with Omni's own server which, by the way, keeps OF on my iPhone synced wirelessly as well. For me, there is no down side AT ALL! Omnifocus is SO well-thought out and their service is without equal!
If you are at all interested, call and chat with them. Believe me, I was skeptical. And now I am probably their biggest cheerleader. But I'm still not going to dump my pc and get a Mac. Maybe someday, but not now.
P.S. this might be a great solution for those who are nervous about keeping their personal GTD system on their work computer as well. I see no reason (other than the cost of the iPad) that this wouldn't solve that age-old problem!