Unit Review: Seers!
Going to try to cover Farseers and Warlocks in this, though I don't have at on of experience with the latter. Farseers can be a vital lynchpin in your Eldar army. I see alot of players don't use Farseers or warlocks competitively, instead going for autarchs so they can more easily reserve deny.
Farseers can give you the edge that you need to win as Eldar. Need to re-roll to wound against a certain squad to make sure it dies? Doom. Need to make sure the shots hit? Guide. Fortune is useful for turning a squad into a bullet magnet, since it lets you reroll saves. Want to get rid of a power fist in a squad so that you can assault it easier? Mind war. And being able to stick a LD 10 character into a squad never hurts.
Taking most of those powers is prohibitively expensive in most cases though. It's a wiser decision to take Eldrad, since he comes with all the powers and wargear, and has a better statline. Otherwise most people just take a single power, ala doomseers or fortuneseers.
One of the most dreaded Eldar units used to be the jetseer council. Warlocks on jetbikes accompanied by a Farseer on a jetbike, using fortune every turn so that their 3+ saves are rerollable. Their witchblades wound everything on 2+ and with doom you can reroll that. They count as str 9 vs vehicles too so, not much trouble popping even landraiders in melee. Each warlock can even take a singing spear and a psychic power that is equivalent to a heavy flamer, if you want to ensure you can take on hordes easily. Such a unit will usually end up costing you atleast half of the points of your army though.
Basically, with proper use of Farseers, you can make more strategic decisions without having to worry as much about the dice betraying you. For example, a Dire Avenger squad with guide, firing on a squad that is doomed hits on 3+, and rerolls to hit, then wounds on 4+ most of the time, and rerolls to wound. 32 shots total if you use bladestorm. I've found the average amount of wounds vs infantry to be atleast 20-25. Even if it is a space marine squad, most of them will die. Very useful for getting rid of trouble units.
As it currently stands, I think that the Farseer is the most useful Eldar HQ second to the Avatar of Khaine. And they work well together too. I usually follow my avatar around with a seer, casting fortune on him every turn so that the firepower put into taking him down is wasted.