Jason Kottke shared some hard-to-wrap-your-head-around facts, extracted from the book The Ends of the World by Peter Brannen:
A rock larger than Mount Everest hit planet Earth traveling twenty times faster than a bullet. This is so fast that it would have traversed the distance from the cruising altitude of a 747 to the ground in 0.3 seconds. The asteroid itself was so large that, even at the moment of impact, the top of it might have still towered more than a mile above the cruising altitude of a 747. In its nearly instantaneous descent, it compressed the air below it so violently that it briefly became several times hotter than the surface of the Sun.
And this one:
[…] in the Yucatan it would have been a pleasant day one second and the world was already over by the next.