The Corpse Cart is quite an interesting unit and something very different to the rest of the Vampire Counts units.
For starters, it does not act like a chariot which you might expect. Instead it shares the same rules as monstrous mounts (although there is no mention of it being a large target so we will assume it is not, irrespective of it's bulky appearance). 
The cart comes with 3 options. You can pay 25 extra points to give it Balefire or the Unholy Lodestone and you can use it as a mount for a Necromancer thereby providing your more vulnerable necromancer with a bit more protection from missile fire.
Balefire
This is useful against enemies with spellcasters within 24" of the cart who suffer -1 on their casting rolls (this doesn't include their rolls to dispel). This provides some additional magic protection for the cart and, equally importantly, any of your nearby units.
The Unholy Lodestone
For a Vampire Counts army not facing powerful enemy magic users like High Elves, Lizardmen, Tomb Kings or other Vampires the Unholy Lodestone is probably a better option.
The Lodestone enhances the power of the Vampire Counts key spell Innvocation of Nehek. It means that successful castings of this spell raise 1 additional undead model.
If playing in a games tournament, you could of course take 2 carts, one with the Unholy Lodestone and another with Balefire. Your best choice would then probably be to place this carts within 6 inches of one another possibly behind units of skeletons and zombies so they can benefit from the carts and, in turn, provide the carts with some ranged weaponry protection.
The corpse cart already has a degree of protection which is just aswell because it has a mere 3 wounds of undead "life" in it. Thankfully it has regeneration and a 5+ armour save. Obviously, any necromancer mounted on the cart does not share these benefits but you will be hoping any missile fire hits the cart and not any necro sitting on top of it.
Miasma of Deathly Vigour
Now to the best bit and the real reason to take a Corpse Cart into battle. This power level 3 bound spell allows a unit within 6 inches of the cart to strike first in combat. With Zombies, Ghouls or Skeletons this is no great benefit but deploy Grave Guard in close proximity and you have a potent combat winning combination. Obviously, such a bound power will be easy for the enemy to dispel. So to make it more effective you should take Vampires tooled up with the Master of the Black Arts to give them an extra 2 power dice and as many bound items as you can get your hands on. This will place your enemy in a dilema as to which spells he wants to dispel and allow you the chance of casting Vanhels Danse* in case your Miasma of Deathly Vigour spell is not effective.
* Vanhels Danse has the same effect as the Miasma of Deathly Vigour spell once the target unit is in combat.
Is there anything else ?
Well, the Corpse Cart itself can attack in combat but will probably not do much to win you a combat with 2D6 (so an average of 7) strength 2, weapon skill 2 attacks.
The Corpse Cart Summary
If you decide to use one or more Corpse Carts deploy them near the core of your army and always near any Grave Guard who have great weapons. Also expect to spend a lot of points empowering your army with spellcasters and bound items.
An army with Corpse Carts will, I would have thought, want to base it's tactics to some degree around the use of the carts. Because of the points cost involed in making them truly effective you will lose out on close combat power elsewhere. So, I guess it's a choice between whether you want your Vamps to concentrate on magic or be more combat orientated.
In my mind they are a handy addition for a reasonable-ish points cost (75) and add an interesting tactical option but are by no means essential. I expect most people will include them in their early games because the Corpse Cart comes in both the Spearhead and Battalion boxed sets.
Note: you will not benefit from the always strikes first rule when fighting High Elves who also share the same rule and have a much higher initiative value than the undead.
Next up... Bat Swarms - which will be a much shorter post !
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Important Errata:
One of my blog readers (Robert) has pointed out something I overlooked, which has a significant bearing on the use of Corpse Carts in combination with necromantic magic.
Vanhel's Danse Macabre not only gives the Always Strikes First ability to troops in combat (like the Corpse Cart bound spell Miasma of Deathly Vigour), it ALSO allows the spell's target unit to re-roll missed rolls to hit. This typically would mean that the enemy might choose to dispel the more dangerous Miasma before the Cart's bound spell (although we should also note that Miasma is a bound power level 3 spell while Vanhel's is a power level 7 and a bit harder to dispel with dice, unless the enemy has a dispel scroll)
Please bear the above in mind when planning tactics.
All credit and thanks to my blog reader Robert.
