I bought the iPad version of Omni Focus a few days ago. For over 2 years I resisted buying the iPhone version because I thought it was too expensive for an iPhone app. I've been using Pocket Informant and Appigo's ToDo (both the iPhone and iPad versions of each).
Before purchasing Omni Focus for iPad I read over 80 reviews of it on iTunes, I watched the video on the Omni Group website and I even watched all 10 videos on the Mac version.
I then decided to give the iPad version a try, and after using it a few daysI absolutely love it, love it so much that I even bought the iPhone version today so that I could have everything with me at all times. When I finally get a Mac later this year I will get the Mac version as well.
One of the key selling features in the iPAd version to me is the Forecast view and I hope that gets incorporated into the iPhone and Mac versions. I also love the fact that everything can be synced via MobileMe. Pocket Informant and ToDo sync through Toodledo, and although good it's not as intuitive. In addition there were things about PI and ToDo that I either liked that the others didn't have or something that one or the other didn't do at all. OmniFocus has it all and the iPad version made understanding GTD a lot easier for me. I have been a long time Franklin Covey user and just trying to grasp GTD over the past 2 years has been a challenge, but watching the videos on the Omni Group website and using the iPad version brought it altogether for me.
I was apprehensive in spending $60 for both the iPad and iPhone version, but now that I have and have used both programs, even just a few short hours today for the iPhone version, I am happy that I did and that the cost was well worth it. Syncing from the iPad to the iPhone took just a few minutes via Mobile Me and so far has been flawless. What more could I ask for.
Later this year, when I finally get the iMac or MacBook, Omni Focus for Mac will definately be on the short list of Mac software that I must have.