> Has anyone with decent (5+ years) experience built a non-trivial new feature in a production codebase quicker by letting AI write it?

I would say yes. I have been blown away a couple of times. But find it is like playing a slot machine. Occasionally you win — most of the time you lose. As long as my employer is willing to continue to cover the bet, I may as well pull the handle. I think it would be pretty hard to convince myself to pay for it myself, though.