Mostly for use cases which are isolated and with a technology I'm not very familiar (like rust proc macros) and which are hard to Google.
I'll give it a system prompt in the spirit of "you assist experts with developing software. Be brief and assume expertise".
I've found it to work well for smaller, contained problems or one off scripts where the alternative would have been to do it manually or not at all. Getting there 80% allows me to start them in the first place.
Another random example: Recently I needed a script to transfer some secrets from one k8s cluster to several others. It took about 3 minutes with GPT 4 and solved the problem within one iteration. There is probably a one liner in bash to do it but I don't know it of the top of my head;)
Copilot massively improved the quality of my logging and commenting
