The most important thing a task management application must do, is stay out of the way. You have to be able to trust it without devoting inordinate time to tweaking it.

I have been using OmniFocus for most of its extended Beta period, and despite its beta status, it has been impressively stable, especially with regard to data integrity. OmniFocus (OF for short) seems usable alone, but it really shines as part of the GTD approach. The documentation in the released 1.0 version of OF is parsimonious but adequate; it's not an introduction to GTD (that's the role of David Allen's book) but it does explain how to use OF.

The designers of OF struck a near-ideal balance between flexibility and simplicity. For example, text formatting of tasks, projects, and task groups is limited, to reduce distractions. Frequently-used settings to control the visibility of items can be saved and quickly accessed using "perspectives." Integration with other apps is natural and helpful. And finally, its organizing capabilities are what you'd expect from the developers of OmniOutliner.

Over the past 20 years I've used several different approaches to task management, and GTD together with OF is the best I've found so far.