I used Things and OmniFocus for a while.

I was pretty psyched about OmniFocus' hierarchical tasks when it came out, but what I ended up doing was organizing complex projects with OmniOutliner, which can aggregate numbers better. I simply pick out the more immediate tasks and add them to my task manager in a flat structure.

I love Things, it really strikes the right balance of features and simplicity. The thing is almost all of my tasks were at my computer, so GTD-style context was not used much. In the end I just needed a todo list. I'm sure other folks need to juggle contexts more often than I do, but personally I haven't had a need justify the overhead at the moment.

I think a major appeal for the current GTD apps are the offline mobile versions. They let users take their apps to actual different contexts, away from the computer.