I've wondered that myself, Nicebernards. Do some iPhone 5's work, are we a minority?
I've tested it with two people (one a Genius, one an ordinary consumer) who are happy with their iPhone 5 and said they don't have any problems with call quality. I ran my test procedure on both of their phones, and their handsets both failed. Those phones exhibit the exact same drop-out for low volume signals as every other iPhone 5 I've tested. I've tested it on three U.S. carriers, and at least six different handsets. All have exactly the same behavior on my test suite - 100% reproducable solid drop-outs of recorded speech that is clear and intelligible on iPhone 4 and other phones.
My conclusion is that the "problem" is present in 100% of iPhone 5's, but the vast majority of people simply don't notice it or aren't bothered by it. If you set the volume high enough, and talk to people who speak loudly, there's not much of a problem with drop-outs. Those of us who make lots of conference calls or talk to people on speakerphones may have a different, less satisfying, experience.
For me, the iPhone 5 was unusable as a business phone, and I returned it. I'm using my iPhone 4 happily, and waiting with anticipation for a fix so I can try again.