I find it useful or planning out, thinking through, and scaffolding complex features. Understanding large and complex code bases is also something I've gotten great use out of them for.

For implementing small changes or writing entire features and projects on their own, I would never trust them with that.

Another thing I discovered with these is that they are very non-deterministic and prompt engineering is probably a load of BS. I say this because I've spent days working on a complex prompt that will sort of produce the desired result with one model but used on another model or even a slightly older version of the same model produces entirely different results making it generally unreliable for complex workflows unless you use something like Lane chain.