I used, and liked, Things, but I replaced it with Task PRO for two reasons: support for repeating tasks and support for sub-tasks (nesting). The nesting feature makes it so much easier to manage lots of complex projects and to-do lists.

Task PRO's ability to copy tasks and their subtasks is a huge time-saver — I set up a list called Templates and used it to create to-do lists for common tasks – packing a briefcase for a business trip, giving a presentation, developing a web page, writing a proposal.

I wish backup were easier; I miss Things' ability to schedule a reminder ahead of schedule for a due date, and I'd like icons that were more meaningful, but other than that, I'm very happy with it.