Thanks for starting such an interesting thread! First, let me say that what I value most about the program are TATL upgrades from Y fares (because that's what my employer will pay for). Given that, I guess the best days for me were the XM days (in other words, before PY). Don't get me wrong, I do think the current J pods are superior to the big blue ones. But back then my upgrade success rate as 50K was around 75-80%, which is much higher than nowadays where my success rate as SE is less than half that. The food was better in those days too.

As others have noted upthread, there are so many personal factors to consider. Back in the XM days, flying J was still a novelty for me. Now, it's lost its luster a bit as I can afford to pay for J on non-work trips, and as AC continues to devalue the product.