I'll mirror some other anecdata here: Not finding Fable to be amazingly godlike at actual coding, but it does seem better at planning, architectural thinking, and reviewing code. Used it to think through some longer form refactors that involve some product decisions and changes, and found it to provide more thoughtful feedback. However that's just my subjective experience, and I don't think it's provably that much better to make me want to go pay for API pricing when the free trial is over.
My plan is to make hay while the sun shines: get some planning in over the next week or so, and just let Opus take care of it when I get to actual implementation.
My experience as well. I quickly stopped trusting Opus to build foundational abstractions because it would almost never to them well and instead would end up chasing into rabbit holes and building overly complex and ugly solutions.
I think Fable is an entirely different experience. It has much better taste, and is better at balancing features versus complexity to a point where I currently trust it to make novel design changes. I still verify it of course, but with Opus I would throw away the solution most of the time while Fable mostly gets it right.
Personally I think the verdict is still out on if Opus & co are actually worse, or the rate at which we move with these tools now is faster than we're used to for managing tech debt and compounding complexity with rapidly built software.
If nothing else, using the smart model for planning to hand off to the previous gen for implementation still seems like a useful pattern.