I've just done AA (77W) LHR-MIA (coming back on BA 744).
Absolutely loved the AA experience in Biz. Cabin crew were superb and the product is significantly ahead of BA CW imo.
We also sailed through MIA immigration. No queues at either the machines or the desks. Although bags took 25 mins to appear on the carousel.
I'll be opting for AA J where possible from now on.