I recommend something very simple and
outliner based, because that sort of philosophy will be useful once you start with OF, and importing will be easier from something simple, too. I've been using
Todoist, which isn't super-GTD strict, but it is an outliner based philosophy that is deceptively powerful and elegant. It scales with what you need to. Simple lists are a breeze to fire off (unlike most web apps, it has
full keyboard support). And working with a system that is based on lists within lists is good practice for the way OmniFocus works. If you really need to work in contexts and plan in projects, like the book, then Todoist will not work for you.
iGTD is an Entirely different philosophy; more along the lines of ThinkingRock, where each task has much more attached functionality, and almost feels like a discrete component. Projects do not feel like items in a bigger list, but singular buckets, wholly unconnected to anything around them. If the line between task and project is something you find to be a bit blurry, or you need a lot of interaction between multiple projects; iGTD's interface gets in the way, I think.
Further, I found its presentation UI creates an overly complex process because it is encourages one to "fill out the form" since it is sitting right there. Due to its "second stage" complexity (pointed out by gofast), and probable difficulty in synchronising with OF when the time comes to transition, I'd recommend not dipping too heavily into it. Of course, one should try out different philosophies. There are a lot of people that prefer to task-heavy approach that ThinkingRock and iGTD provide, and applications like OF will probably not suit them as well.