It won't always give you a perfect answer the first time, but it's much better than memorizing the manual or interpreting a forum discussion. Haven't used it for ffmpeg, but lots of other command lines.
I would far rather look at the manual or a forum discussion, because then I know I'm getting something real. With LLMs, odds are decent that I'm getting something which doesn't actually exist, but it sure would be nice if it did.
I've had someone post a problematic ffmpeg command into a prompt to ask why it wasn't working. It didn't work so well. By the time that someone rejiggered their prompt, I had found the issue.
