I think it depends on the person, and the boot / shoe. I've not worn shoes for so long I can't actually remember what they feel like, I always wear DMs, New Rocks, or Converse (or wellies if the occasion demands!).
I bought brand new New Rocks in Whitby, and wore them all day for a couple of days, while there was a bit of discomfort it wasn't bad and they seem fine now (though I've only worn them for clubnights since, as I wear converse to drum and Black Sheep follows our band practices.
My last New Rocks (the ones with the dead zip I need to get sorted) were broken in by going for a six mile hike along the canal one evening, and while they did indeed rub one heel raw and bleeding that day, they were fine afterwards.
Btw, re combat boots, I remember a program about new recruits to the Royal Marines (back in 82 - they wondered on-air if they'd qualify in time to be sent to the Falklands, and were dissapointed when they didn't), and their instructor's reccomended method of breaking in new boots was to get them to walk up and down in a bathtub full of warm water while wearing the boots. Then again, these were presumably the same boots that were then found wanting when qualified marines did yomp over the Falklands, so the type may have been replaced long ago.