So, I'm sure you already know, but Dark Eldar have Harlequins in the army list. They are exactly the same as in the Codex: Eldar entry, right down to the stupid lack of a dedicated Transport option. GoGoGadget 4e units!
Really though, this means they pretty much suck. The DE aren't exactly expensive overall, from what I saw, but the simple fact is they are no CSM either, and their seemingly very good design means they won't be hamstrung by running out of FoC before points and forced to waste any anywhere, and thereby take Harlies over super-Wyches, or Incubi, or Mandrakes, or Grotesques, or super-Warriors...
As Chumby points out, they have a truly phenomenal assault range...and they may well be better than Mandrakes. All the same...they suck for Eldar, and they suck even harder for Darkies.
When you have Truebloods...I mean, Trueborn, running around with a shitload of Blasters and whatnot, instead of mediocre combat troops (that don't score, like Wyches...) that carry a pair of THREE INCH Meltas...yeah. Suck, with a capital City. :-/
Chumby, don't forget to send me that document. *Nudgenudgewinkwink* Say no more. ;-)