Their role is a fast cavalry only one. Do not use them to attack rank and file enemies (unless charging highly ranked units in the side or rear)
They should be used to hunt down skirmishers, missile troops and artillery crews.
If armed with spears and bows they are quite costly when you consider the fasr movement rate of the High Elf army. You should only really need to use them against fast moving enemies like rat boys (Skaven), the green slime (Lizardmen) and Woodies.
Note: don't bother using them against armies with a lot of cavalry. Although, you might manage to avoid combat (with a move of 9 inches) if the enemy cav does catch you then you're dead meat.
