I bought a couple of pairs of REI or Novarasomething bike underwear. Essentially they are lycra boxer briefs with a chamois. I wore them maybe four times under regular shorts. Horrible. They creep up your thighs. They don't breathe. There's no extra room in the crotch, so at every stoplight, I'm digging around like David Wells on a humid day at Fenway. (That's an unpleasant image, I'm sorry. But accurate.)
I'll believe you that regular bicycle shorts are comfortable. But I just can't.... Are MTB with built-in chamois really worth it?