Wracks are an elite choice that can be troops if you have a Haemonculi in your HQ slot. As an elite choice they are cheap, and might be required in your lists to get pain tokens on the tabletop, however it is when they are troop choices that they really shine.
WS4 BS4 S3 T4 I4 A1 Ld8 Sv6+. They are very flexible in unit composition being 3-10 wracks. They come with two poisoned weapons (4+), night vision, power from pain, altered physique (start with a pain token). I was really surprised wracks got an initiative 4 and such high WS and BS. For every 5 models one wrack can replace a poisoned weapon with a Liquifier Gun (S4 APd6 template). They can also take a Raider or Venom for a dedicated transport.
One Wrack can be upgraded to a Acothyst, who can then take a large variety of weaponry. Stinger Pistol, venom blade, mindphase gauntlet, hexrifle, scissorhand, flesh gauntlet, agoniser and electrocorrosive whip.
To start, I really don't like Wracks in small units. I know you can have 3 or 4 in a unit, but the only reason you would do this would be to take a small personal escort for your haemonculi or archon to fit into a venom. You would be short changing yourself as you need 5 wracks to get your liquifier gun. Then you are taking up a very important slot in your army.
5 Wracks in a venom. While cheap, I don't find this small of a unit that effective. We are Dark Eldar after all, and even with feel no pain we are not that durable. This is the smallest unit I would consider, and then only if I need to fit out a venom, which I prefer warriors in.
The idea of Wracks is to hold a position. 10 Wracks with Feel No Pain in cover will do just that. Remember that you are more of a defensive unit and you do not come with grenades. No assaulting into or out of cover. What you do get though at 10 wracks is two liquifiers that should be just what you need to route something coming at you. If that isn't enough, you do come armed with two poisoned weapons. Just what the doctor (Haemonculi) ordered when facing big bugs.
If you are fighting out in the open, wracks with a second pain token can really do some damage. Having Furious Charge gives you 3 attacks at S4 on the charge at I5. This means you are hitting marines on a 4+ and wounding on a 4+ with a re-roll (since your poisoned weapons are at a strength equal or higher than the toughness of your opponent).
Your Acothyst. While flesh gauntlets are nice (instant death on a 4+), a lot of people are drawn towards the hexrifle. It is one of those remove from play sniper rifles, and the saving grace of this weapon is that its an assault weapon 36" AP4 sniper. With the hexrifle a model wounded must take a wounds characteristic test or be removed from play. While a lot of people are talking about hexrifles, I don't see them being very reliable, but hey if you have nothing else to do with the unit cause everything is too far away, by all means fire a hexrifle. There are other options as well, scissorhands for instance are nice.
I like Wracks a lot. I will be using them mostly as a defensive unit, probably to protect webway portals and objectives for the most part. They will do good in assault if you are not going through cover, and I like their template weapons (liquifiers). With two liquifiers wracks can be good to remove tougher units out of cover. Don't forget how far you can reach with your wracks when mounted on a raider. Raider movement is 12" deploy at almost 3" and a template is 8"'s long. That's almost 23" of coverage for your liquifiers. A proper rotate could give you another 3" but that is not reliable depending on position.
Possible Combination against incoming transports. Trueborn with blasters or other tank killing units, and Wracks with 2 liquifiers. Pop that transport and flame away with what ever comes out. I havent practiced this, or done it yet, as my games lately have been on my opponents board edge and not near mine. Any time though you can pop a transport and kill everything that survived the wreck with your next unit's shooting, it will make your opponent cry.
