As the title says. What brought you to Commorragh?
I'll start, and I have three reasons, first one being that they have good models. Really good models, I'd argue that they are some of GW's very best, as far as armies go. I like the hard edges and flowimg armor panels, and the spikes are a nice, welcome touch.
Secondly, the Dark Eldar (usually) have a freedom that the uptight Craftworlders do not, which makes for some quite interesting kabals and characters. And there is absolutely no loyalty or sense of caring for your fellow man, if you're fighting with someone at your side it's because it's safer to have someone with you, nothing more. It's kill or be killed, and almost everything you get for yourself in the Eternal City is earned the hard way. I have an odd respect for that, even though it usually means a lot of knifework.
Thirdly, and probably most important to me, is that they're really evil, it's almost impossible to top what the True Kin do for entertainment, the Dark Eldar really ram home the 'grim darkness of the far future' in a way many other things can't. Sure, working on an imperial forgeworld and being doomed to a short, miserable life sucks. Getting conscripted to the Imperial Guard is also bad, but nothing compares to what the Dark Eldar will do, your fate with them is a nightmare that, in certain cases, never ends.