Erm, some people DO take it seriously. Rashad Khalifah, who started the trend, was actually murdered in his own mosque by hadith-loving muslims because they took offence to his ways and the fact he called himself a messenger of allah.
I think more offense was taken at his claim to be a messenger and also his saying that the Quran had been altered or changed than his rejection of hadith. (Hey, he was accused of molesting and having sex with a young girl, so maybe he was a messenger in the Muhammadan tradition!) The Submitters, his group of hadith rejectors, are kind of nutty, and the people supposedly involved in his assassination were just as nutty - they weren't mainstream ISNA types by a mile. The ones I've known have been very racist. They reminded me of the neo-Nazi groups. A lot of those Quran only sites are run by them, without their name or their 19 theory on them, or it's run by a group that broke away after Khalifa was killed, and they're still loony. I've only met a few normal seeming people who rejected the hadiths...