I echo everything dws said above.
I am a heavy OmniFocus user and use it daily (Mac and iPhone versions)--I really don't know what I would do without it. Apple's Reminders app doesn't even come close to doing what I need as far as task management, which is really the crux of the issue. There are lots of "to-do" lists, some are even free, including Apple's Reminders, but that's not what OmniFocus is. OmniFocus is a task management system that follows a published and well-defined methodology (Getting Things Done®). It also works seamlessly with Siri (You can tell Siri "Remember to ask Bob for the updated TPS report", and "Ask Bob for the updated TPS report" appears in your OF inbox).
Here's one simple example of a feature that OF has that Reminders (nor Things for that matter) does not: OmniFocus has the ability to set a "start date" for a task, so when I suddenly remember that I need to ask Bob for that TPS report, but Bob is on vacation until next Tuesday, I don't have to try and remember this, nor do I have to keep looking at a list of tasks that contains "Ask Bob for TPS report" even though I can't actually do anything about it (which causes mistrust in and ignoring your list of tasks). With OF, I can still add the task, but set a start date of next Tuesday and Bam--it's off my mind, it's in a trusted system, and I don't have to keep looking at it for a week when there's nothing I can do about it.
That seems like a small thing, but when you compound several (or dozens) of Projects, each with dozens of tasks, the ability to do things like this, as well as keep track of these tasks across multiple "Contexts" (I won't bother going in to that in this post) makes OF a very powerful tool, and one that I know can be relied upon to show me what I can do at any specific time or location.
Most people don't need OF and can get along just fine with Reminders or some other free to-do list. But some people need something with more power and features.
By the way, I still use Calendar to schedule meetings, events, etc. OF doesn't replace a calendar, and actually integrates very well with it--on any given day, it will show you the tasks that are starting that day, due that day, and on the same screen, shows you your meetings that it pulls from Apple Calendar.
Sorry for the long post--hope that was helpful.