I have (kind-of) adopted the Nozbe application for these reasons:
Nozbe is a goal setting and tracking application, complete with a calendar, and multiple other categories to organize your to-dos. There is a tab for categories, and another (separate) tab for projects within (or unrelated to) those categories. This iPad (and I believe iPhone) compatible application also allows you, with payments, to add other members to your account. These people can view your goals and achievements, as well as being able to make theirs visible to you. However, in an attempt to be extremely organized, the app seems to have sacrificed some of it’s user-friendliness, and I found myself jumping tab, to tab, to tab, trying to get everything in the place that I wanted it. This application, while being a good fit for some people, was not necessarily a good fit for me, as I ended up wasting most of my time trying to figure it out and organize it correctly. So, although it was not a terrible application, I think I will stick to my good old planner notebook. It has a calendar, daily spots for writing in goals/appointments and such, and is chalked full of sticky notes for my own organization. I am simply more of a pen and paper type of gal!