I have had some of the best and worse experiences with US Immigration. Mostly I have to say that I have been treated very very well indeed. About 4 years ago going through Bangor, Maine on a delivery flight, I had the worst experience of my life. It made me seriously think about ever accepting flights to US again. I wrote some nasty letters regarding this agent. Later I received an apology from Bangor Airport Manager. I thought this was nice. I have been through there since and never had a problem . Wonderful fuel stop for a thirsty B727 wether you go East or West.
Probably the best experiences for me where in San Fransisco where I was taken off the line by a INS agent who saw my Crew tag on the briefcase and took me to the crew and diplomatic line.
Miami is nice too. Last December it took me about 1 minute to clear Immigration and another to clear Customs.