I've lost count of the number of transatlantic flights I've taken. My experience is largely with Delta, KLM and Virgin. All are adequate and comfortable and have decent inflight entertainment and food. American Airlines are dreadful, I flew them once and never again.
You have to collect your checked luggage at the first US airport if connecting because you have to clear it through customs. It's not because of 9/11 paranoia.
My experience is that JFK is horrendous for immigration arrivals and connections. Atlanta is fine for returning back to the UK as your luggage will be checked through to your destination on the way back. Detroit is by far the best for arriving into the US and making connections. It is also a nice terminal when flying Delta/Skyteam.