Whether on foot or mounted on a jetbike, the Farseer is the lynch pin of the Eldar army. If I am playing my Eldar army, I will always field at least one of these guys. With a plethora of nifty psychic powers and a badass Seer Council Bodyguard, it's hard to see why you wouldn't field one of these guys.
Due to the specialist nature of the Eldar army, I feel that the Farseer is necessary due to his ability to augment any one of those units that is likely going to fail at its objective. Those Banshees not going to do enough damage to Terminators? DOOOOOOM! Do you need that Land Raider gone this turn? Guide your Fire Dragons and you'll guarantee that it'll be a pile of slag. Powerfist about to charge into your Wraithlord? Mind War him and chuckle as he bleeds out of his ears.
Most Farseer's run Doom and Fortune as their psychic powers - this gives reasonable flexibility with offensive and defensive support - allowing you to help out in just about every situation. Try not to buy more than two psychic powers since all it does is make your Farseer super expensive and you don't get to use the third or more power.
Jetbike or no? I find the jetbike to be particularly useful due to the +1 Toughness and the 3+ armor save (without which, Psycannons would leave my Farseer a smoldering crater) but understand that you will be walking around with a 160+ price tag in a fairly high cost army as it is.
- Runes of Warding: BUY BUY BUY BUY BUY! In a world where Jaws of the World Wolf, Murderous Hurricane, Lash, Fortune, Doom, etc etc are powerful game changers, you need to be able to shut them down.
- The Warlock bodyguard. They can add a whole lot of hurt in the form of Destructor Spam. 10 Point heavy flamers? I'll take that. They add a lot of ablative wounds to your Farseer and, when combined with Fortune, are super hard to crack due to a 4++ rerollable save (or 3++ if you buy jetbikes). Now, the Jetbiking these guys is going to be a costly endeavor. They will each cost more than a terminator and in order to be effective, you're going to need 5-6 of these guys. But man, is this a killy unit...
If I'm going to talk about Farseers, I feel a little obligated to talk about this guy. King of the Farseers, he gets 3 psychic powers a turn (if he's not in CC), can use the same power twice due to his staff, wounds on a 2+ with no armor saves and allows you to redeploy some of your forces after the both sides have deployed, meaning you can have first turn and a reactive deployment. He's a powerful character - use him wisely since he is darn expensive. Can't have a jetbike unfortunately. :(