Just to confirm what others have said, they take your fingerprints and your photo every time you enter - we have been through immigration at Tampa, JFK and Atlanta since November, and they definitely know from your fingerprints when and where the last time you entered the US was.
In terms of delay, the airports all seem to be a lot more organised on this these days. In fact, I have found that the busier the airport in terms of international flights, the better orgnaised they are - in JFK and Atlanta, we waited no longer than 5 minutes. In Tampa, it can be more like an hour.
The moral of the story is to be prepared for it, but if you can, book the seats closest to the front of the plane as you can. Provided they disembark from the front, you should be closer to front of 300+ people. Plus make sure you have completed the Visa Waiver form (green) and the immigration form (white) correctly. Otherwise a mean immigration man will take great pleasure in sending you to the back of the queue.