I'm not sure if this was covered here, but given my AC membership was up for renewal, I started investigating this card seriously. While others have mentioned that this car provides a benefit of FULL AC membership, that in fact is not true, at least not for another month or so. here's why...
one of the biggest/best benefits of the AC is the lesser known rebooking service whereby AC angels pull up distressed pax lists after getting a call from the Tower or noticing a delayed inbound. The way the pull the list is by originating flight and member. The guest roster shows an ADM flag next to passengers who are true members. This helps the club actually go through the list, make calls to people traveling in, etc.
After asking a few different AC agents, it turns out that Amex and Citi entrants do not have this flag, and therefore have no way of coming up on the roster in the case of such a query. As such, as of now, the admirals club membership still has distinct advantages over the citi one.
One note was that after much digging, I received a call from one of the agents at SFO. She said they've flagged the issue after it was reported, and that AA IT will be working on a solution over the next few weeks to fix it so that Citi members do show up with a flag that can be queried.