Looking at how the LispWorks implementation — originally actually foreign to OS X — is gaining ground on OS X (the previous version already ran cleanly under OS X, but this one has made further progress), one can clearly see how the distributor of the originally and still macOS native MCL implementation is failing in the market. You can't really put it more positively — MCL is in my opinion to be regarded as a debacle these days. At least the biggest news on their websites for 2004 is from the summer — that they got their mailing list running again. Great. MCL was once the star in the Lisp sky. At Planet Lisp you can find the original article.