I have only flown J recently with AA but I agree with the sentiments above – lounges pretty poor,uninterested crew,average food but very comfortable seat.
One consolation is that J can be much cheaper than BA as I have found it quite easy to buy a cheap Coach ticket ($700) here on the US web site and upgrade to J with $350 and 25,000 AA Advantage Miles. Even if you assign a value of $500 to the 25,000 miles that still gives you a RTP J ticket at around $2400. You may not get the upgrade straight away but I have been on a Wait List twice and it came up both times 36 hours prior to departure.
Edit: Flying 77W JFK-LON-JFK in October