Kevin de Young, gave this great paper about impassability at this recent American conference (T4G).
Kevin points out that impassability is out of vogue because our culture places a high premium on God suffering with us. However the doctrine of impassability describes God as perfect without needing any new experience, knowledge or glory. This of course doesn't make God dispassionate, emotions are, says Kevin according to Vanhoozer, "construed values" which God feels according to the changeability of his creatures. Furthermore the beauty of impassability is heightened by the incarnation and the sacrifice of Jesus, God becoming man and as man, suffering with us. Otherwise we don't need a High Priest who is tempted in every way because God already struggles on larger scale. In addition abandoning impassability opens the door to God not knowing the future, making the doctrines of grace, just a program of self-help for a suffering God rather then a generous overflow of unwarranted love.