I think that CK is a little overrated to be honest (over and above ExPlat) - my partner is CK and I haven't seen any "wow blowout" moments when we've been travelling together, especially on AA metal. Premium flights (LHR-JFK, LAX transcons) are often fully-booked in the premium cabins, so it's by no means a guarantee that CK upgrades clear, and pre-boarding happens anyway with Group 1 and Group 2 (on AA). The biggest benefit that she gets is the IRROPS treatment, which is brilliant, and the tight connections transfer. I've you're on AA metal, then they will do absolutely everything in their power for you to make the connection. Doesn't always happen with ExPlat / OWE.
That said, I've heard that GS, iO, AP and some of the other invite-only programmes are wonderful. Perhaps for another forum / discussion.