After flying UA for years, I am in London now on the first trip of a Trial Preferred challenge. I've been really pleasantly surprised by everyone from AA/US I've interacted with. I had to rebook BOS-JFK-LHR because of a weather delay that would have led to a missed connection, and the agent was competent, pleasant, and got me what I needed quickly with no problems. It's a really nice change of pace from United!