I've been on a quest for the perfect task/ time management system for years. I concluded quite some time ago that the goal will always remain elusive but the questing is valuable. Although many trials of new systems get discarded, I tend to keep the bits that work for me and incorporate them into my ongoing personal system.
I commonly hit two issues: most systems (e.g. GTD) seem overly rigid, and flat task lists, even with categories, don't support the way my task list grows. My tasks often grow hierarchically. The task starts life as something like "Draft plan for perpetual motion machine". But as I start working on that task it spawns sub tasks like ""Review past work in this space", "Meet Fred re: his Project X work" and so on.
Hierarchical task management systems do exist but are rare. I have used ListPro, and I have used MS Project quite a bit. Project is a good hierarchical task manager but its features are overkill for most personal task lists. They both have their own rigidities, and don't integrate with Outlook in the right way. Ultimately I actually want my tasks to show up in Outlook. Because I want to see them along with my calendar and be able to allocate them to slots on my calendar, get reminders etc. Outlook 2007 rocks for all this now but it still doesn't do hierarchy...
With the combination of OneNote 2007 and Outlook 2007 I now have a system that works very well for me. Here's how I manage my task list and workflow now.
Now my work flow typically looks like this: