I pasted some headers (some of them very long, in multiple chunks) and asked the bot to write me a summary of what the API does, and what could be used to make it better. Answer were spot on. Free documentation! If you also paste the source, it can give you a documentation for a particular function as well. Same for rather complex Makefiles.

Otherwise I do like others, I use it as a quick stack overflow for uncommon APIs (to me). Or completely random questions, knowing the answer might be dicey.

It has replaced google at about 90%, I only use google to verify, and not all the time.