Yes it is legal. Sold at most of the gun shows. Available via mail order too.
is it all its cracked up to be
It IS safe to drive around with 20-30 pounds of mixed tannerite in your car. A crash won't set it off. You'd have to be driving 1363+mph and have to be equivalent to the general rule of 2000fps that a rifle round would have to go in order to set it off.