Things forces you to work a certain way. Omnifocus lets you specifically tailor your way of working.
To cut a long story short, Omnifocus has a steep learning curve as it does a lot of stuff and it takes a while to figure out how you want to use it for your own workflow. However, once you've done that, you won't want to leave Omnifocus and other applications will appear lacking.
Also, sync between OF and the iPhone app is brilliant, and both the mac and iPhone applications are continuously in development, UNLIKE pretty much every other to do list / GTD style app out there.
You may find OF frustrating to begin with. But I swear you won't regret it.
Greetings from Sweden.