I turned Copilot on in January and haven't turned it off. That doesn't mean I think it's everything it's marketed as. It's useful for a specific slice of the work and actively unhelpful for the rest — and after six months I have a pretty clear picture of which is which.
Where it actually saves time
Boilerplate is where Copilot earns its subscription. Writing a new REST controller, a data transfer object, a test fixture — anything with a predictable structure that I know by heart but still have to type — Copilot gets it right 80% of the time and I accept the suggestion without reading it carefully. That's real time saved.
It's also surprisingly good at writing the boring middle part of unit tests. Set up the mock, call the method, assert the result. Three lines I would have written anyway, now I just Tab through them. Fine.
Where it makes things worse
Anything that requires understanding context beyond the current file. Copilot doesn't know that we have a custom exception handler, or that this service has a specific transaction pattern, or that we deliberately don't use Optional in this codebase. It suggests the generic Java way, which is often subtly wrong for our specific setup. I've merged Copilot suggestions that looked right and failed in integration tests for exactly this reason.
Documentation and comments are worse with Copilot than without. It writes plausible-sounding Javadoc that describes what the code does rather than why it does it that way — which is the only part of a comment that's worth writing.
The mental overhead nobody talks about
Reading generated code takes almost as long as writing it, especially if you're going to own it. I've caught myself accepting a 20-line suggestion, then spending 5 minutes verifying it's correct, and realizing I could have written it from scratch in 3. The time saving is real for simple cases and imaginary for complex ones.
My current rule: if I can picture the full implementation before I start typing, let Copilot write the first draft. If I'm still figuring out the design, turn it off — the suggestions become noise that shapes my thinking in the wrong direction.