Funny you ask - just yesterday, I (reserve card holder) was refused entry at MEX AMEX Centurion Lounge. Agent insisted that it was "only in the USA" that Reserve card holders could access the AMEX lounge. I didn't believe this, but didn't fool with arguing as I only had a few minutes and the Aeromexico lounge was next door where of course I could and did get in. But I was annoyed nonetheless...