Ok, I've used a Coven Throne in couple of games now, and I must say I am stunned how well it performs. Here is the build:
Aura of Dark Majesty
Talisman of Preservation
Now, that is a lot of points in one model that cannot join units, but it is very resilient. I have not lost it in a battle so far, though the Vampire himself dies in almost every game. Both the rider and the mount have Ward Save 4+, and the rider has Armour Save 3+, which is nice. Pallid Handmaidens score an odd wound or two, so does the Vampire himself if he lives that long, and ghosts beneath rarely do anything. It has good impact hits, and as already mentioned has better combat potential than most chariots. The bound spell it can cast is also ok, sometimes can work wonders but it sucks that it lasts only in the turn it has been cast.
But above all there are two things that really make it shine. First of all, it has ethereal movement, making it suffer no dangerous terrain tests, so it is able to position itself as it likes, which brings us to the other big quality. Battle of Wills. On paper it does not do much, or so I have thought. Lowering BS is nice, but lowering WS by 1 is crap. Trust me, it is not. In almost every game I have tried to combo charge it with a Varghulf (but I guess it works great with many other things too, say Fear Incarnate or Screaming Banner) and at one point opponents fail their fear test. With a Ld 10 Lord nearby and hopefully a living Vampire with Aura of Dark Majesty, enemies often fail their Battle of Wills by 1 or 2. This means that their WS is 0, so they cannot hit me, and I automatically hit them, and it not only goes for Coven Throne but any other unit in close combat with that enemy unit. It is devastating. Units that are immune to psychology or cause fear or terror, in my experience, have lower Ld, so although they will not fail fear tests, they are more likely to suffer harder from Battle of Wills.
I play against High Elves most often, and it is a single unit they hate most and try to destroy from the start of the game. Just for kicks, I switched the armies with my friend. I had a horde of 30 White Lions, so I thought I didn't need to keep them in range of the Prince as they can shine even on their own. Coven Throne Vampire charged me with his lord nearby. With a Ld7 I failed my fear test, so it went to Battle of Wills. He had Ld 10, I had 7. I rolled 5, he rolled 3. I had WS 0 now, he scored 7-8 wounds, I failed my break test on Ld7 and he ran me over, without a scratch. Name one other thing in Warhammer that can fare that well against Horde of White Lions.
Just wanted to share how thrilled I am with Coven Throne.