I use Omnifocus on my iPad my iPhone and my Mac Pro. I am very happy with it as it provides a unique way to manage my tasks and projects. It works extremely well, syncs with almost anything you can think of, is very stable and on the iPad very polished. It uses the principle of Getting Things Done. You start by creating an action on your Omnifocus Inbox. You can select start and end time, put the action to a context (you can create many contacts, hierarchically too), assign it to a project, or even make it a subtask to another task.
There is a context view where you can see all available actions grouped by context. The project view shows all projects with it's associated tasks. There is also a Perspectives view, where you can see tasks that end soon, that are overdue, etc.
The is also another great feature: Review. You can have weekly reviews of your projects and see if sth is missing or needs attention.
I have also used Pocket Informant's Task feature, but I really prefer Omnifocus, even if it is so expensive.