Haven't hiked it (with a backpack and all) but I did climb out from Bright Angel Campground to the south rim starting at noon, and man, was it hot! (I rafted halfway down through the canyon, then hiked out, but the mules carried my gear.) A shower never felt so good.

But I did want to second Pedal Wench's recommendation of spending some time at the North Rim. Sooooo much better than the South Rim. South is like a city, North is like the country. Hotels vs. cottages. Frantic vs. laid back. Love love love it there.