As a freelancer I too have found it both helpful and interesting from a data perspective to track my time in a detailed way. I've been using Excel for years, plus an independent (from Excel) calendaring system to block off target periods for specific projects.

From the usefulness perspective, tracking my time allows me to a) objectively see where I'm slacking off or rockin' it. Perception can be skewed without the objective tracking; b) view my effectiveness, but only because of how I use my tracking system. If I'm not being effective, I don't enter time for that period. But inspired by OP, I'm going to add an effectiveness metric to my tracking system, and possibly include what my distractors are; c) have a built-in reporting system to my clients, if I need to track hours vs. job for invoicing; d) gain a better sense of how much any given job will take me in advance. This sense has improved dramatically by reviewing my time tracking-data longitudinally.

Thanks for sharing, OP. And thanks to the rest of you who posted your own systems and links to other ones.