Not GP, but I can.
I'm currently working on porting a mid-sized project to a new architecture, new programming language and of course adding new features.
Getting a new feature implemented is quite easy. You spend a few hours brainstorming specs with the agent, then ask it to implement it. This gives you extremely frequent code drops that add a new brick, add a new feature, etc. All of this with 100% code coverage (we also have mutation testing, strongly-typed code, standard and custom linters, etc.)
Then you look at the code. Code that has passed review, generally. You realize that the database schema has been broken silently, and that the agent has rewritten the tests or the golden fixtures to match. You realize that it has made assumptions that contradict the specifications and the product is going to break once it's in the hand of users. You realize that the 100% code coverage is essentially a convenient lie, because the code and tests have been written to make passing easy. You realize that none of the security golden rules have been followed, and that has managed to happen because the agent has somehow deactivated linting.
Why did it pass reviews? Well, because of deadlines. And because there is simply so much code (and so much unparsable/misleading documentation) that it's simply impossible to review all of this. And because things move so fast that nobody understands the CI pipeline anymore, and the explanations of the agent are convincing enough that surely, it knows better than you?
On the upside, bugfixing becomes so fast! Just add a new test, wait a few dozen minutes, and a new Merge Request appears. With equally convincing/misleading explanations, and something else broken.
After ~4 months, we had a bare bones deliverable, which we're now steadily expanding. If we had had to write the product manually, I suspect that it would have taken us at least one year, possibly two. So, that's the productivity increase. The productivity decrease is that what we have is not a product but a glorified demo, something that will work very nicely on the happy path, but on any other path, all bets are off.