I was the subject of this scanning modality twice recently: at LAX and in Mexico.
On both occasions I tripped the magnetometer due to some bits of stainless steel holding my leg together, and the next step, absent any pat down was to proceed to the scanner.
Not a problem really, hold your arms out, wait for the bars to rotate by, turn 90 degrees and repeat. The downside of course, is that it is being used as secondary screening, so when I'm not driving a corporate plane, I'm stuck with it.
I'd be much happier if I could just opt for it up front.