if u have a bad memory.. very important.
if u like to be organised.. important
if u dont really give a rats.. u can live without
if u like doing everything at the last minute, without much prior warning... then dont bother, its a waste of ur time to get one.
They usually have signs up when they're starting to give out student diaries. Probably within the first couple of weeks of the autumn session. Don't get too excited - they're pretty dodge