How much of brain do we really use?

You always hear people say humans only use some small percentage of their grey matter, say 13% or so.
I always ask people how functional someone with 87% percent of their brain missing would be. That usually breaks the bubble of that myth.

Still. The Question: How much of brain do we really use?

Unca Cecil explains:
http://www.straightdope.com/classics/a3_028.html

Someone sent me this yesterday…seems appropriate for the subject.
The Buffalo Theory as told by Cliff Clavin:

One afternoon at Cheers, Cliff Clavin was explaining
the Buffalo Theory to his buddy Norm. Here’s how it
went:

"Well ya see, Norm, it’s like this… A herd of
buffalo can only move as fast as the slowest buffalo.
And when the herd is hunted, it is the slowest and weakest ones at the back that are killed first. This natural selection is good for the herd as a whole, because the general speed and health of the whole groupkeeps improving
by the regular killing of the weakest members.

"In much the same way, the human brain can only
operate as fast as the slowest brain cells. Excessive intake of alcohol, as we know, kills brain cells. But naturally, it attacks the slowest and weakest brain
cells first. In this way,regular consumption of beer eliminates the weaker brain cells, making the brain a faster and more efficient machine. That’s why you always feel smarter after a few beers.