New body-scanning machines have detected hidden weapons like ceramic knives and flagged illegal drugs. But they've also tested the patience—and vanity—of some passengers.

The Transportation Security Administration has rolled out 80 of the machines at 27 airports and so far received 600 comments regarding their use out of about 4 million passengers scanned. Not a lot, and many travelers say they have come to accept the electronic peek under their clothing. But with a goal of 450 body-scanners on line by the end of the year,...