For me it has a big positive impact on two sides of the spectrum and not so much in the middle.

One end is larger complex new features where I spend a few days thinking about how to approach it. Usually most thought goes into how to do something complex with good performance that spans a few apps/services. I write a half page high level plan description, a set of bullets for gotchas and how to deal with them and list normal requirements. Then let Claude Code run with that. If the input is good you'll get a 90% version and then you can refactor some things or give it feedback on how to do some things more cleanly.

The other end of the spectrum is "build this simple screen using this API, like these 5 other examples". It does those well because it's almost advanced autocomplete mimicking your other code.

Where it doesn't do well for me is in the middle between those two. Some complexity, not a big plan and not simple enough to just repeat something existing. For those things it makes a mess or you end up writing a lot of instructions/prompt abs could have just done it yourself.