I usually enter via IAD, but I have entered in ORD several times. I prefer IAD, although my experience with ORD has been good. I do not terminate in IAD as I fly onward to EWR. If my flight arrives in advance of the several UA flights, one can usually clear INS & Customs in one hour. If you arrive after several of the UA flights, I find the delay is with getting your luggage rather than waiting in long INS or Customs lines.

The best airports for clearing quickly are Schiphol & Changi. If you hit it right, you can do it in 30 minutes.