Farseers work great with DE, especially when you've made a significant points investment in your Eldar. They are a force multiplier, and even a simple combo of Fortune and Doom make a blade storming Dire Avenger unit (transported via waveserpent) a very resilient unit, with a heap of dead;y high volume, high accuracy shots that also counts as scoring. You'd be surprised just what a difference 4+ re-rollable saves can mean to a Dark Eldar army, especially when that unit is being brought around in a trusty wave serpent.