I got one of these as well. It usually works really well, although I have had problems two times. In each case, I got off a 747 and was among the first people into the immigration hall. I went straight for the INS Pass machines, which had broken hand geometry readers. By the time I got back into the regular lines, the whole plane was in front of me and it took forever to get through immigration.

When they work, they are a real timesaver, particularly in the NYC area airports (both Newark and JFK have them).