For historic reasons, and because between the two of them I have a unique set of features I can't find elsewhere, I use Toodledo online and Appigo's Todo.app on my iPhone (they sync). The combination is a bit of a kludge, but by now they are burned into my brain. It would take real improvements for me to switch, and I haven't seen them . For example, Appigo's online task app is missing Toodledo's essential mailto feature.
Despite years of use however, I've not done much with Contexts in either Toodledo or Todo . I understand the theory, but in practice it's been too much bother -- until today.
Today I realized what I needed was an easy way to tag small tasks that I can easily knock off when I have a free minute or less -- and I realized a single letter context was a convenient way to do that. So I deleted all the unused Contexts I'd created, and added a new one called 'q' (for quick!). Now, when I'm bored, I filter on q and knock off a bunch of little tasks.
 I could build such a great task app if I had another life or two ...
 Todo syncs contexts with toodledo.