Harlequins.

I feel like they are a bit of a let-down so far. The rules for the Harlequins, Solitaire, and Skyweavers have been released and...meh? Harlequins themselves feel a bit like Wyches, but with some better (and more expensive) upgrades - how good/useful the unit would be allied with Dark Eldar will completely depend on what the Transport ends up like. If it's a mini-Wave Serpent with assault then they may have some merit. The Skyweavers have good rules for a massive price. 50 points a model for a 2 wound, 4+/5++ half assault, half shooting platform seems a bit too much. Each model can take a Haywire Cannon (24" haywire with a blast) for 5 points, which is cheap, but...meh? They're just too expensive at T4... I think I'd rather have Reavers. I can get as many Haywire Blasters I want into a list already, and more than 4 feels like it'd be pushing it (your left in a sore sport sitting with Str 4 AP 4 shots if you're up against non-Mech/Tyranids/White Scars/Whatever).

I was really hoping the rules would be a great complement to Dark Eldar, but it's just not looking that way. Even if Eldar gets a nerf with some toned down rules they still have a lot more going for them (even though the Harlequins are some of the best models GW has had to date).

So the fate of the Harlequins will be completely left up to their transport. If it's a great transport that also sits in the Fast Attack slot then some great times will be had with Dark Eldar allying. If it ends up being an expensive, but beautiful and rules heavy model, then it just won't fit. I'm hoping it's not open-topped - because that'd be the start of a beautiful friendship (not open-topped, but would still absolutely need assault).