I use Beeminder occasionally. The site’s emails normally suffice to bug me into accomplishing whatever I’ve committed to doing. But I only use it for a few tasks for which my motivation is marginal. Most of the times that I consider using Beeminder, I either figure out how to motivate myself properly, or (more often) decide that my goal isn’t important.
The real value of Beeminder is that if I want to compel future-me to do something, I can’t give up by using the excuse that future-me is lazy or unreliable. Instead, I find myself wondering why I’m unwilling to risk $X to make myself likely to complete the task. That typically causes me to notice legitimate doubts about how highly I value the result.