I flew via ATL few times this year, arriving/departing with BA's First and Flying domestic in Economy with AA, using my OW Sapphire.

Both lounges available to BA flyers are not great, but The Club was particularly bad, I actually believe to have caught food poisoning there, was chaotic, dirty, long queue for drinks, the BA area was ridiculous, the food bad and the whole experience stressing. Using the terminal connecting train to get to the Admiral lounge was easy and quick, and the lounge was always half empty (I am guessing it is not an AA heavy status airport), had passable snacking food, some time a reasonable hot dish but free beverage was quite basic and you even had to pay for sparkling water.

Slightly off topic, but as a point of reference, the Flagship Lounge in Miami has been one of the very best lounge that I have visited for the first time this year, and it is night and day compared to the ATL Admiral lounge (and I am guessing also to other Admiral lounges), I believe with an overall bigger difference between than what you get at HT5 between Galleries Club and Galleries First.