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.
Because ffmpeg is built on the Unix chained utility philosophy I find ai is also good at building scripts the use it as well
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 find it helps if you paste in the the ffmpeg manual and get the ai to use that as source. Helps it stick to real params.