Dark Elves are fortunate enough to have a multitude of choices when it comes to their Special (and Rare) choices. Again I will give a nice brief summary of each unit.

Shades. Smaller units of crossbowmen scouting and skirmishing behind the enemy. Great to whittle down the enemy (and the generals sanity)

Witch Elves of Khaine. Probably the best unit for your points. 2 poisoned attacks, 3 due to frenzy, with a hag that can take items from the Temple of Khaine (dark Elf only magic items and such). Plus the ability to take a 50pt banner creates a block of infantry that is not somehting to mess with. only downside? no armor save.

Cold One Chariot. Scythed wheeled chariot being pulled by two fear-causing dinosaurs with a crew that can one again take RXBs? AND you get two for one special choice. Sounds like a good deal to me. One thing though, Cold Ones suffer from Stupidity, lucky for you its at Ld8 or more!

Cold One Knights. Thought heavy cav was cool on horses? Try on dinosaurs. Armer with a lance the riders benefit from +2 Strength on the turn they charge. Again, watch out for Stupidity...

Executioners. Dark Elf Elites wielding the massive, dreaded Draich that can sever an opponents head in one swing. Armed with the Killing Blow ability at WS 5 these guys can be a driving force in you army, and for a very nice point cost!

Harpies. Probably the cheapest flying unit you'll find. Perfect for taking out war machines

Here we go for the specials!

Shades: Can be effective but I'd find them hard to place in a unit. If there's a lack of terrain on the board or someone guesses that your fielding them can leave them stuck on your side of the board or somewhere in position to do nothing productive. However, they can definetly put a hurting on some war machine crews or small ranged units if they get the positioning right. Verdict is still out for me.

Witch Elves: Completely overrated unit in my opinion, as is any unit with Frenzy. A smart general will simply send a useless unit like fast calvary or skirmishers and send those babes on a wild goose chase the entire battle. Not only that, but they have no armor at T3. Hell, a couple units of my horse archers can whiddle these girls down to nothing real fast. The rumors of a ward save in the next edition make them more appealing though, I personally greatly dislike Frenzy. No go for me.

Cold One Chariot: Chariots can be extremely nasty, and especially if they are pulled by cold ones. However, 1 cannon ball or Str 7 hit can ruin your day (instant death for the chariot) and the stupidity can sometimes find itself effecting you at the worst of times (right before a charge perhaps?)

Cold One Knights: Probably some of the most powerful calvary in the game as far as offensive power. You give them the banner of murder and suddenly you've got this rather effective killing machine which with the mounts makes them a unit that will also be effective in subsequent rounds, unless all horse calvary units. Expensive and stupid do make their toll though, but overall I'd take them.

Executioners: I mean comeon, a WS 5 unit with Killing Blow?? These guys can be very potent but I think they can only be taken with a CC character leading them. While the killing blow is effective, this unit with a hero or lord level in among them will make their killing time not really as potent. However, I'd try to work them in if I could.

Harpies: Same logic behind fast calvary apply to flyers like these guys. However, these guys can just go over like it's no big deal. As they can also be an effective screening unit (protecting the heavy hitters from missile fire) makes them invaluable in my opinion.


Done!

I also have a problem fielding the witch elves for the same reasons that scurrdi mentioned. As for chariots and cold one knights, I enjoy putting them in but the stupidity can reall hurt at times ( Trust me been there done that). I I got to where I was fielding two units of cross bows and two units of dark riders to offset my bad luck with magic and stupidity rolls. i hope to fair better with the new army book.