I use it to journal how all my time is spent in a given work day- for both work related tasks and non-work tasks (ex. getting coffee, lunch, conversations, etc.). Org mode is the only GTD software package I've seen that makes this possible without getting in the way. It just needs one small thing that keeps it from being perfect.

Those who use it the same way I do (as a log of how all time was spent in a given day), there's no easy way I can see to make quick adjustments to clocked time between activities. Every day I will forget to clock to a new activity at some point. By the time I remember, time has passed. For example, I come back from a meeting and begin to work on a project. 20 minutes into it, I clock in. But I must then manually adjust both the previous activity and the current one so that they are accurate and don't overlap in the agenda view. This can be cumbersome, which seems to go against the overall idea of org mode. All that is needed to correct it is either a new fn and key binding, or a prefix arg to org-clock-in that allows you to enter an adjustment (in this example, 20) to subtract from the previous clock's out time and current clock's in time.