Yew is medically useful, but can also be dangerous. Taxol is the active ingredient in several cancer medications, breast cancer being the one that gets the media coverage.
http://science.jrank.org/pages/7450/Yew-Medicinal-uses-yews.html Concentrations in one tree can be hundreds of times higher than in another !
The English long bow is made from yew wood. This was the decisive military weapon in Europe for 300 years, surviving well into the age of gunpowder. The trees were nearly all cut down in Britain during the reign of the long bow. Some of the oldest examples are in church yards where a taboo against their removal has protected them.