Used LAX regularly and never had a problem, longest queue was about 30 mins and still had to wait for baggage...
Be polite, co-operate, dont' ridcule the US or their customs/ways, answer questions they ask and provided you are 'legal' you won't have a problem.
At least the US has separate lines for their citizens unlike the UK where we share the queue and wait with the other EU passengers whilst non EU passenger whizz though the short line.
HTC