I'm a little biased caused I developed it. But I got tired of trying to mold GTD into general purpose to-do apps like OmniFocus and Things so I made Purpose. Any program will have it's strength and weakness. It's all about the many design choices we make when we're making these programs. I made a decision right up front to not worry about making something that would have general mass appeal but instead focus like a laser beam on making something that works for GTD as seamless as possible. Nothing is going to be perfect. For example, I preferred the way Things does tagging (for Contexts) over OmniFocus's contexts.
It's true that you can do GTD with anything (such as pencil and paper) but the problem is that when you're starting out you're still learning a lot of what GTD is and the subtleties of all it's parts. In this early stage I think the wrong tool can make GTD harder to pickup if you'e not clear on it's concepts and how they inter-relate. In many ways, paper and pencil are better when you're starting out because they're less constricting and make you think more about the GTD methodology which is more important than any particular tool.
Anyway, don't want to get banned by the forum gods for mentioning my product so I'll stop there. Just throwing my two cents out there cause it was asked.