I'm not sure if I would use org-clock unless I were already programming in Emacs, but since that's mostly what I'm doing, it works great to keep track of contracting time.
It's difficult to know though, because I started using org-clock alongside org-mode, and if you combine time-tracking with outlining, journalling, todo/project management, and a calendar, all of it well integrated, it's hard to imagine switching to something else.
EDIT: also, just remembered that one reason I quite using Toggl was that clocktables turned out much more powerful for analyzing/reporting my hours.