The standard harlequin troupe comes in sizes of 5-10 in your valuable elite slots. that puts them in high competition with your trueborn or incubi. This alone will make them rarer to see on the tabletop. The do not come with a transport option either, and this to me was very surprising. I had assumed they would get a venom transport option since it was in the harlequin experimental codex the venom first appeared.
WS5 BS4 S3 T3 W1 I6 A2 LD9 Sv- is the standard harlequin stat line. Death Jesters and Shadowseers also come with the same stat line, and the Troupe Master will get an extra attack and leadership point. You will notice no save. Apparently being almost naked (wyches) gives you a better save. However Harlequins come with a 5+ invul save from their Holo-suit.
Their special rules are fleet, dance of death, veil of tears (shadowseer only). Dance of death gives them furious charge and hit and run special rules, while veil of tears is a psychic power that makes an enemy unit wanting to shoot at you roll 2d6x2 like night fighting, in order to shoot at you. If they cannot spot you they lose their shooting for the turn. This is why a shadowseer really should be in every unit of Harlequins.
Wargear they bring to the table is a close combat weapon, shuriken pistol, Flip belts (ignore difficult terrain), Holo-suit (5+ invul save), and Hallucinogen Grenades (shadowseer only and counts as the whole unit having plasma grenades). Up to two can replace thier pistols with fusion pistols, which are a 6" melta gun. Finally any model can replace their close combat weapon with a Harlequin's Kiss which just gives a close combat rending weapon. Once you upgrade with a kiss though you are up to a staggering 22 points per model equalling the same as an incubi.
Not taking a shadowseer upgrade really defeats this unit. You need one, not only for veil of tears, but for the grenades. Death Jesters on the otherhand carry a shrieker cannon which is a nice 24" S6 AP5 Assault 3 pinning weapon. He is only 10 pts with his cannon, so why not.
The Troupe Master is a little more expensive, however he can take a power weapon or kiss for free. Now why they made both free I don't know, since really the power weapon is the obvious choice.
While I like Harlequins, it was the transport option that turned me away from them. Yes they can run forward without much fear from being shot, but they are very slow without one. They are a turn 2 at the earliest unit, except for maybe the death jester.
Good options for using them include;
Attaching a Haemonculi or Archon in order to get them safely to midfield. Round two webways, however I find this option too late to matter. Get those webways open round 1.
Spread out in the open to give units behind them cover saves while shooting at the harlequins is hard to difficult with veil of tears.
Load them up into a transport and fly foward. Remember that because you embarked, the raider cannot move flat out. Aethersails can help get you where you want to go.
As a unit, rushing them through midfield cover. They should be in cover anyways, as this is where this unit excels. With two fusion pistols though it does give you some extra short range firepower in a true S8 melta weapon. Two Fusion Pistols and a Shrieker Cannon are a nice combination of weapons that you can use to your advantage in small units. 120pts for 5 with the 2 fusion pistols and the Death jester. 150 with the Shadowseer included.
I cannot recommend them all upgraded to harlequin kiss's as then you are paying the same cost as an Incubi. However, fielding them with Incubi is an idea I would like to try. Fielding them tandem with incubi, who suck in cover when alone, might work well with the Harlequins running through the cover.
Overall I do not think the Harlequins are bad, I do find them wanting a little though. They will be situational when designed into army lists that need them. I do like them being able to stand out in the open to give cover to more valuable units, however, is this really what I want out of an elite choice unit?, probably not. Besides giving cover to other units, which will work, I like them moving through any midfield cover with ease. However, really any unit can do this, and this is why I find them a little disparaging.
I would really like to hear other uses for them, as I would love to like this unit a little bit more.