I have, and it was amazing.

You could quickly make the changes you want not worry about what else breaks, then run the tests. The tests that you expect to fail would fail, and you fix them. Then you find other tests that you didn't expect also failed; you'd review the impact of your changes on those parts of the system and make appropriate changes until the tests were happy.

Because the testing was so thorough (and quality), you had high confidence that nothing unexpected was broken.

It was "move fast & break [tests], then fix them and deploy" :)