The iPad app is good. I have all three, and use all three, though the iPad app gets the biggest chunk. The Mac app has a lot of functionality that is not present on the iPad or iPhone, and frankly isn't likely ever to be there (a scripting interface, for example), but as you are coming to the iPad app from a position of never having had those features, you won't know what you're missing :-) The syncing works very well with Omni's free service. Some of the "missing features" that you don't get on the iPad are things like being able to change a bunch of actions in the same way simultaneously. You can still make the changes on the iPad, but it takes longer. I actually use both iPad and Mac apps together, because the iPad app is superior in some ways, so I might make one change to a project on the Mac, sync, and make the next one on the iPad! I don't use the iPhone app so much because my iPhone is ancient and I almost always have the iPad, but when I do, I'm frequently reminded of how good it is, too. One nice thing about buying from Omni is that they add a lot of new development to each major version before putting out a paid upgrade.
The Siri integration
looks very cool on the iPhone 4S (and is an example of something added for no additional charge).
Buy it when you can spend some concentrated time getting started. If you decide it doesn't work for you, you can contact Omni for a refund if within 30 days of purchase. You might also take a look at the OmniFocus/GTD white paper
put out by Omni and the David Allen Co., just to make sure you don't have incompatible ideas about how GTD should work. The paper is illustrated using the Mac app, but the approach is the same.