I got Fable to run overnight and I woke up to a working prototype of a very complex feature. And then I did it again for another complex feature the next night.

The code still took weeks to clean up, but it worked and was correct. It felt then, and still feels, like a big step change on very hard problems. These are problems I would previously expect to take a month or longer to implement.

I have also noticed Fable can handle much more nuance when reasoning through writing and research, but that is harder to quantify.

Not OP, but to me it initially felt extremely proactive and energetic, just powering through roadblocks with ingenuity and enthusiasm. After it came back I was constantly getting refusals and downgrades for things Opus had been doing. I’ve written it off for my use cases and getting by just fine with Opus 4.8 and now 5.