I went through O'Hare recently, where they've installed a battery of these devices. The way they had it set up, it was used as the secondary screener for people who got called out for random searches. Guess who got called for "random" searches in the machine? Pretty much every hot chick in line. Now, this observation was purely anecdotal, so take it for what it is. But the coincidence was not lost on me, or on anyone…
I was at O'Hare this summer and noticed the exact same thing. All the dudes ended up going through the metal detector, and most of the girls went through the full body scanner. Then I got really upset when I saw them selecting a young teenage girl to go through the scanner as well - perverts.
It would be very interesting to take data on male/female ratio over course of a few hours.