Is the 9 box grid worth the effort?

It is worth it for what it does to the conversation, not for the accuracy of the placements. The grid gets managers into one room using one vocabulary, makes their disagreements visible, and shows how performance and potential are distributed across a whole company rather than one team. Few alternatives deliver that. It is not worth using as a promotion gate on its own, as a forced ranking, or as a prediction about any individual. If you run it, narrow the potential question to readiness for a named role in a stated timeframe, require evidence for every rating, check the distribution by gender, tenure and manager before signing off, and give every placement an expiry date.