This, so much. My code since using Copilot is easily ten times better tested than it was before, and I wasn't especially lazy when it comes to testing.

Given 1-2 hand-written unit tests, Copilot can start filling in test bodies that correctly test what's described in the function name. When I can't think of any more edge cases, I'll go prompt it with one more @Test annotation (or equivalent in another language) and it will frequently come up with edge cases that I didn't even think of and write a test that tests that edge case.

(One great part about this use case for those who are a little antsy about the copyright question is that you can be pretty darn confident that you're not running a risk of accidental copyright violation. I write the actual business logic by hand, which means copilot is generating tests that only interact with an API that I wrote.)