As part of an effort to further systematize my personal and professional organization, I’ve recently started using Remember The Milk. I first started using it in 2008 and recently started using it again. I am finding the functionality (and the iPhone app) of the service impressive and quite flexible. That all said, it does have lack one feature that Google Calendar has: the ability to have multiple reminders. Since some tasks or activities are complex (e.g. require coordination with other people, require advance planning etc), I find multiple reminders helpful.
Assuming one is motivated to be organized (Seth Godin points out that lack of motivation, rather than lack of knowledge, often undermines productivity efforts), it is crucial to combine systems (Getting Things Done by David Allen is the classic here) with the right tools. I have used paper based agendas at various points, but at this stage I prefer to use digital systems. What I find great about seeking to improve one’s productivity and organization is the immediate benefits one receives. Executing a series of items on a properly organized to do list is satisfying.