I do greenfield in fluid dynamics and Claude doesn't help: I need to be able to justify each line of my code (the physics part) and using Claude doesn't help.

On the UI side Claude helps a lot. So for me I'd say I have a 25% productivity increment. I work like this: I put the main architecture of the code in place by hand, to get a "feel" for it. Once that is done, I ask Claude to make incremental changes, review them. Very often, Claude does an OK job.

What I have hard times with is to have Claude automatically understand my class architectures: more often than not it tries to guess information about objects in the app by querying the GUI instead of the data model. Odd.