I've never used it and don't know that I've had a beer made with it.
Amaranth is a plant that grows like a friggin weed. It grows almost every where, Native Americans, Africans, and Aztecs have used it for God knows how long. It's little seeds pop like pop corn, and thats how it is usually eaten. In South America they make a beer like beverage out of it.
I think it has good uses as an adjunct. So unless I was making a beer for someone with Celiacs Disease, I'd only use it for a 1/3 or less of the grain bill. It lightens the beer like corn would, only I like the flavor it imparts.
Here is some basic information I put on Ehow.com.http://www.ehow.com/how_7586438_brew-beer-amaranth.html