The job I'm asking my task manager to do is probably the most important of any of my software choices. In essense, it houses my second brain.
I've been using OmniFocus several times a day, on various devices, since 2009. It's probably the most sophisticated task management tool that exists, and I would happily pay triple what Omni Group charged. Here's why:
Many task managers allow you to form your own system for working with tasks. I feel that this is intellectually-lazy. It creates a mess of due dates, priorities, inconsistent folders, and tags that don't mean anything. I'm attracted to tools which are built around a process or a philosophy. OmniFocus employs David Allen's Getting Things Done method, a method which has informed so much of my own. This prescriptive approach is opinionated, brave, and highly effective.
The Omni Group have shipped versions of OmniFocus for Mac, iPad and iPhone. This means that my task system is ubiquitous, whether taking notes, studying in depth or in a pocket. This is important because it makes it easy to capture and review tasks, but more important than that, easy to habitualize the capture and review of tasks.
I also really like the fact that OmniFocus makes it relatively simple to build custom perspectives, giving you a particular view of your data. I have perspectives for all of my likely situations, displaying available tasks deferred to the contexts which constitute those situations. This means that if I tap the "Study" perspective, I see all tasks which are appropriate to that situation. I'm working on a paper describing this process in more detail. I also recommend Kourosh Dini's excellent book, Creating Flow with OmniFocus.
Where OmniFocus really shines is its ability to conduct a review of captured tasks and projects. It's easy enough to capture information, but that's useless unless you have an easy system to review the right information at the right time. Reviewing the actions in a particular situation is made easy with perspectives, and freshly-captured tasks can be seen using the Inbox view. However, to conduct a regular review of projects, there is a dedicated Review mode built into the iPad and Mac version of OmniFocus; and coming soon to the iPhone. This makes it quite easy to go through every outstanding project, check that it has at least one available or deferred task, then say whether the project is current, paused, done, or to be dropped.
I've yet to see any task manager implement this mode as effectively as OmniFocus has; yet it is essential for keeping your system maintained and up to date.
I have a lot of respect for The Omni Group as a company; and that is important for the vendor of such an important tool. I've found them to be very conservative when it comes to features that could compromise the stability and security of data, and their post-sales support has been consistently excellent. They are more expensive; but this is a situation where a high price is a good thing, because it inclines them to look after me as a customer.
There are lots of task managers on the market, but the only other tool which I think is worth considering is Todoist. Unlike OmniFocus, Todoist supports many more platforms, including Windows, Android, and has a native web version. While not quite as a focused as OmniFocus it is reassuringly expensive which gives me confidence about the intentions of the company behind it.
This is an area where you want to spend money. You'd buy yourself the best pacemaker if you could; by the same logic you should buy yourself the best task manager. I depend on OmniFocus. It works best when you buy into the process that it's built to support; unlike other task managers it won't work for you if you go against the grain. That said, if you're trying to implement the habits I described in The Productivity Habits, or similar systems, OmniFocus will serve you very well.