Large-scale deployments in Ireland (1.4 million out of 4.9 million people) and Germany (12 million out of 80 million people), anyway. In Ireland it has lead to the discovery of a number of positive cases, which is positive; if they'd been discovered at all through conventional means that would have taken longer and the people concerned could have been spreading it for longer.

There has also been successful deployments in Taiwan and Singapore, but AIUI they don't use the Apple/Google API and are far more intrusive.