I flew to New York about a month ago and I was impressed how quick the immigration process was, even with my usual problems getting the machine to take all the fingerprints, on one hand, at the same time.
Leaving Heathrow,my iPad had to be put through the security check on its own,when I went through the US security for a connection,it could be left inside my hand luggage,but, they wanted my CPAP machine to be placed in a clear plastic bag and separate to the rest of my hand luggage.In neither case was there any further inspection after the X ray machine.
Always check the TSA website, and the individual airline's websites for the latest information on what is allowed in hand baggage. Virgin and Delta had basically the same dimensions for hand luggage,but, I have seen bags advertised as cabin baggage which was larger than the allowance.
Keith