Went through immigration/customs yesterday (I think- little bit confused on the time right now!!) and was very impressed. Fastest we've ever been through. I read this thread before we left UK so we were preparing ourselves to be pushy and ask for fast track, but it all came to us!
We saw the signs saying U.S. citizens and fast track to go right when arrived at the hall, but like sheep we just blindly followed the crowd into the main long queue. We were cross with ourselves for doing that but we all just walked through without stopping a couple of up and downs until we reached a point where there was a man stopping each family (bit like FastPass!)
He asked 2 questions 1: Do you have Esta's? 2: when was the last time visited. Those who said never/can't remember/ages ago/more than 5 years to question 2 were directed to carry on through the long queue. I was instantly prepared for our family and answered 1: yes 2: 2 years ago, so was instantly directed to the fast track machines. The man was very quick directing people and I got the impression that if there was any doubt he just sent them straight on in the main queue. It also appeared that if there was one member of the family who hadn't been before or a long time ago even though rest of family had been recently the whole family were directed to the main queue.
My suggestion here would be that if that was the case for anyone, to split up when going to this man - perhaps those who go fast track can go on through to get baggage which is in full view directly behind the immigration booths. I would only do this though if you were 2 or more individual families together as one big family. Basically split up into households is what I'm trying to say!
He asked the 1st person from each family group that reached him, so was not concerned how young anyone in the family was or how new the passport was - it was all based on the last visit.
There were more fast track machines than I was expecting - didn't count but they took up a whole rows worth of a queue doubled up (reminded me of self checkout at supermarkets!). They are basically for customs (our immigration chap said we didn't need the custom form per family we filled out anymore). They give you a fast pass to immigration!!
They were easy to use as well: one person in the family slides their open passport picture page into a slot, confirm your details on touch screen when it finds you, say no to everything for customs (providing that is the case!) place four fingers on pad and have photo taken. It asks you then if there are any other members of your family so you just keep repeating until run out of family members! You then take slips of paper the machines give you and join a nice, short, quick moving different immigration queue to everyone else and carry on as normal.
I didn't check how long it all took, but it seemed really quick - I would guess 20mins. When we looked at the main queue once we had got through immigration, that was fairly short, so it obviously speeds that queue up as well.
Moral of my story is to ensure you have your 2 answers prepared when getting to that man - any uncertainty and you'll be sent to the main queue. Incidently there were people who said no to the esta question - unbelievable! We never saw the outcome of that!