FWIW, I have been using Bonsai, Cando and Pop! for almost a year and am very satisfied. I use Bonsai to set up projects under "life focus areas" and tasks and next actions as children of projects. All tasks are assigned context using Pop! to ensure standardization, e.g. @call, @office... and all NA's get linked to the palm todo.
Categories are projects, tasks, NA, successful outcome, role, focus area, S/M project and projects in planning. Project and NA categories are color coded so I can filter on the two and see immediately which projects lack NA's.
Using Cando to view NA's, I can set a "view" to show items filtered on text, so I can view all items which contain @office or @home or @call to see NA's by context. This way, I just select a alreay defined "view" and whamo, all the items with a specific context are shown. I don't really use the categories in todo applications.
The only thing I really miss is having a way to print out my calendar with tasks, e.g. daytimer format, so I can do some creative on paper thinking.
Hope this is helpful,