First of all Eldrad has Divination which is a pseudo version of Strategic Redeployment. Before first turn, you can reposition D3+1 Units within your deployment zone. This can turn out to be very usefull if your side deployed first, giving you a chance to get LOS you need, or get into cover from something.Next up are his psychic powers
Doom- Target a non-vehicle unit within 24". Re-Roll to wound against that unit. So any power weapon squads become particularly deadly. Twin linked weapons are double guaranteed. This can be used to take out those annoying unit like Nob Bikerz and TH&SS terminators.
My Favourite in Apocalypse: Eldritch Storm: Don't bother shooting this at infantry. Not really worth it. What you want to do is shoot it at vehicles. Even better is super heavies.
Here is what you do: Use Divination to get withing 18 inches of a superheavy. You then Eldritch Storm it in your shooting phase to spin it around so you see its rear armour. Great way to kill them first turn. Works best if you have first turn (and if you have allied Tau with Rail Guns). And Eldrad can do this from withing a vehicle, so a Falcon can increase his chances of survival and give him mobility to reach a new vehicle each turn.
Fortune: Use this as needed to keep squads alive. Great if a squad looses a transport or is going to be vulnerable for some reason.
Guide: Use this to kill vehicles with your lance weaponry or as needed otherwise.
Mind War: Take out enemy characters, especially ones that grant abilities like Rites of Battle or extra attacks to squads they are around.
He can use 3 of the above per turn so long as he is not in assault, and may use one twice if he wishes too (spin 2 vehicles with eldritch storm).
Put Eldrad into a Seer Council with some Warlocks and they become even more potent, or if you dig up the old 13th Black Crusade Book and use and Ulthwe Council with the Avatar in it, they are super deadly.
His wargear is also great
Runes of Warding makes all enemy psychic powers harder. Runes of Witnessing make all of his easier.
So Runes of Warding along with guys with psychic hoods as allies becomes rather potent, meaning fewer get through and then they can be blocked still by a hood.
He wounds on a 2+ with a power weapon. Has a 3+ inv save and a 3+ save from perils. A witch blade and shiruken pistol to boot just add some flexibility.
Basically, you want him to stay out of combat to spin vehicles and buff squads, all the time protected by warlocks. Though they can easily engage in combat with all those whitch blades.
Good Allies to be Taken with: Tau, SM, IG, and Chaos. I know the last isn't fluffy, but it is practical.
So there you go. This guy can be a huge benefit. I recommend taking him if you have a small contingent of Eldar.