Thanks to its hypersonic motor (HSM), AF operation is fast and precise, with a minimum of hunting. a 3-position switch on the lens barrel lets you limit the focus range, with settings of Full (no limit), 0.52m - infinity (for distant subjects), or to 0.38-0.52m, for close subjects. It's pretty common on macro lenses to have a limit switch that keeps them from going all the way to closest focus, reducing AF hunting with distant subjects, but I think this is the first time I've seen a lens that had a similar function to support close-up work as well. I did find that the cameras hunted more at very close shooting distances, but it wasn't clear whether this was the fault of the camera or the lens. They did always find a valid focus point eventually, and the worst hunting was perhaps a second or so at closest focus.
And, speaking of closest focus, it gets very close indeed. A minimum focal distance of 38cm may not sound like a lot, but keep in mind that this distance is measured to the film/sensor plane of the camera, not to the front element. Measuring from the front of the lens, closest focus is only about 10cm, or four inches. At that distance, the frame width was about 22mm on our EOS-20D test body.