I've used at-least one beast pack in every Dark Eldar list I've ever ran, but lately I've started to become more and more disillusioned with them. Don't get me wrong, they are a great unit and I'll try and keep this article balanced before putting my two cents in at the end.
The most common pack size I use is:
3 Beastmasters, 5 Khymera and 4 Razorwing Flocks
This clocks in at 156pts. Sometimes I will expand it by adding another beastmaster and 5 Khymera but the 3/5/4 pack is the most common one I use.
The unit itself is pretty self explanatory; the Khymera, with their 4++ take all the strength 6+ shots and any special weapon attacks in combat, the Razorwings soak up all the small arms fire and provide rending, the Beastmasters exist solely for leadership. 1 Beastmaster can get a special weapon but with only one attack it's not really worth it.
In an army of really fragile vehicles and infantry a well constructed beast pack is surprisingly durable. My standard pack contains 28 wounds and can suffer up to 5 wounds before loosing effectiveness.
The beast pack is also often the only assault element in my army, sometimes they are run in tandem with a unit of whyches but in recent weeks I've been running a large pack solo.
The rending provided by the Razorwing Flocks also means the pack can have a go at popping tanks on occasion and the large space the unit takes up means multi charges are fairly easy to pull off.
This large space can be a liability however. If your deployment is quite compact, a common occurance with Dark Eldar as you want to hide behind any cover you can get, the pack will take casulties from exploding vehicles. Don't kid your self about this, your vehicles will explode and the surrounding troops will take damage.
Leadership can be a problem. The unit is leadership 8 if Beastmasters are around ld 5 if their not. This can leave them quite vunrable to tank shocks and loosing combats. This can be mitigated by attaching a character to the unit but then you loose the bonuses being a beast.
Most importantly for me is the fact that the Beastpack is surprisingly slow. Yes they can fleet, and yes they can charge 12 inches but the fleet is random and therefore unreliable and you actually need to be in charge range to get that extra movement. A clever opponent will deny you this.
Beastpacks also don't come with frag grenades so that naturally high Dark Eldar initiative is wasted as soon as you go near any difficult ground. This can then result in you loosing combat with all the leadership problems I mentioned earlier. The Baron is quite commonly used to solve this problem by bringing a phantasm grenade launcher to the party. He also gives his unit stealth which can help with survivability and with his jump infantry movement can slingshot the beasts forward.
However using the Baron this way means your not making any use of all the benefits he gives to a unit of Hellions. This in my mind is not using him to his full potential, so I'm skipping that step and giving a mob of Hellions a try.
Hellions move 12", can shoot 2 poisoned shots each if they're not going to fleet, come with combat drugs and furious charge (through pain token shenanigans) and with the Barons bonuses should be all round awesome.
I'll do another post on how I'm getting on with them in the next week or two.