I find the Apple reminders app too limited. The value of OmniFocus to me is the multi-dimensional view, specifically the ability to take a Context view across all Projects or a Project View across all Context.
There is Siri integration with OmniFocus on iPhone and iPad as well as a need capture tool in the web browser. I use that piece to quickly move items from my inbox to OmniFocus as part of my GTD process.
Here is a post I made in another website with details