I got to use it for a couple of days on Pro. Feature implementation actually fit into subscription usage, but then it went on chasing its tail fixing bugs it introduced and burning through about $15 in credits. It got the job done in the end, though. I'd say it's good, better than Opus, but I found that both GPT 5.4 and 5.5 are way better than Opus.

So Fable is as good as GPT 5.5, if not better.