I got the Express Connection service in LHR last week. My incoming flight from AMS was delayed and my connecting flight to JFK had less than one hour to departure. There was a person waiting for us at the end of the jetbridge with signs with our last names and our connecting flight number. There happened to be 4 of us on that flight connecting to the JFK flight. They had 1 agent for every 2 people (so 2 agents total).

The agents gave us the bright orange ticket holder and said keep it visible at all times. They escorted us from T5 to T3, then escorted us to the front of the T3 security line, then escorted us to the gate. They then escorted us to the front of the line at the gate too.

The service seemed decent, but the only real benefit to me was getting to cut the security line and being escorted with one of the Jonas Brothers who was also on the flight to JFK. Oh wait, the latter isn't a benefit since I'm not a teenage girl.