I used it a few weeks ago to track down a weird regression/bug in Firefox. A few years ago I used it to track down a regression in Wine.

That's probably the most important case: large complex codebases with lots of changes where "wtf is going on" isn't so obvious from just the code.

I've never used it for any of my personal projects or even at my dayjob, because they've much smaller with far fewer changes (relatively speaking).