In the Russian Kettlebell Challenge book, it states that "serious athletes can use kettlebells seven days a week. You'll get a lot less sore."
Do you mean people who are already serious, full-time athletes who practice kettle bells seven days a week will get less sore from their athletic activities? Or less sore than kettlebells will make them if used less frequently?
Or do you mean that people who ASPIRE to this level of physical excellence should use a 7-day a week (with widely varying intensity) program?