I don't know much about the other ones, but I think you'd have to say PyPy has been a success. Although to be honest, I don't know why it would be better to mod

I don't know much about the other ones, but I think you'd have to say PyPy has been a success. Although to be honest, I don't know why it would be better to modify CPython vs. just using PyPy -- the JIT speedup does come with some tradeoffs (memory usage, warmup times), so it seems better just to leave that decision up to the user?