I have primarily used Fable 5, Fable 5.1, and now Opus 5.5. I never use shift-tab to enter plan mode personally. My general workflow is chatting with the machine to work out fiddly bits, then /plan to get everything in one document, and reviewing the plan.

/plan is still useful, I still need to review what it's going to do and still make revisions. But there's two phases: hammer out key design decisions then write and amend the document.