I have some medium-sized open source projects that I write or contribute to on github.
1) my daughter's form-based web project--loads of content, loads of pages with various inputs. She had only partial content, and ChatGPT took it upon itself to fill in the rest. The content was quite good. I never did get satisfactory layout, however.
2) refactoring of a god-object from another repo. It took awhile, but Claude got there much faster than I would have. Added additional functionality at my request. Now published and working. Generated documentation of classes and methods.
3) conversion of a jsx website to a tax (JavaScript to typescript). I figured this would be a disaster, but nope. Claude did the conversion (about 50 components and additional methods) in a few days working with it. Also, all documentation, including release notes. And tests.
My experience:
1) treat the agent like a talented junior developer who is very fast and quite thorough.
2) it will get confused and forget things. An instruction.md file really helps to prevent regressions
3) it will get stuck in loops and go down rabbit holes at times. Test and commit often so you can rolllback breakage.
4) proceed incrementally where possible. Small, discrete steps work best
5) ask the agent to analyse/explain before doing
6) don't be afraid to ask it for suggestions; they can be quite good (it did a nice job improving the layout and color scheme of my website, for instance).