Last few times Ive been there is just an almighty queue at Customs which took about an hour and a half to get through.
I've had some interesting experiences clearing US airports :s usually there are like 3-4 lots of 100m long queues of people waiting to clear at the big airports and if you get stuck behind that good luck
usually as a white European though you won't get stuck on the big queues.
They can be ultra fussy about the visa stuff (probably less of an issue now with the changes to the visa waiver) - once had to go through it all again because apparently I used the "wrong" colour ink.
Oddly had a few times though where one of the security guys has waved me over - taken a quick look at my passport then escorted me through - kind of weird walking down past long queues of people - they seem to randomly do it to people who are on their own. (I half wondered if they used facial recognition for people who'd been through before but probably nothing to that).