Harlequins are not quite as powerful as they were in the 4th edition, but if use in conjunction with doom on high toughness targets, a 10 man squad with kisses can do an awful lot of rends.
If you get all 10 with kisses in on a charge, against a
WS 4 T 4 enemy they will pump out avg. 13.3 1st roll savable wounds + 6.65 reroll savable wounds and 4.4 1st roll rending wounds + 2.2 reroll rending wounds for a total of 20 savable wounds and 6.6 rending wounds.
If it was
WS 5 T 5 enemies the numbers would be (6.66 + 4.44)11 savable wounds and (3.33 + 2.22)5.55 rending.
Now everyone knows that harlequins are vulnerable to mass low strength fire. We can prevent this as best we can by putting them behind the rest of our army and equiping them with a shadowseer.
In my list which seems to do well against many lists I employ a few other units to help my harlequins get to the juicy enemy units. I use Eldrad, the Avatar of Khaine, a troop unit of 10 wraithguard with a spirit seer, and a back line of 4 dark reapers including an exarch with tempest launcher and crack shot.
The tactic with this list is to set them up how I have pictured below. Move them up in line with the harlequins normally 6 or so inches behind the front line of wraithguard. As they move up fortune the wraithguard and avatar, after the first or second turn you may want to fortune the harlequins if it looks like they will see combat. For Eldrads 3rd psychic power use doom to doom whom ever you will be charging with the harlys, or whom ever is most beneficial. Once the front line of wraithguard is charged or is able to charge, the harlequins counter charge and hop around the combat with their hit n run capabilities.
The dark reapers are mainly for those units that are dug in, especially since you can fire your exarch at targets that are not even visible, and get two small blast temlpates at
str 4
ap 3 that reroll to wound. This guy alone usually kills between 10 to 20 power armored enemies a game.
With this core list, you can add what ever else pleases you, normally some objective grabbers like jet bikes, fire prisms, or falcon/waveserpent with fire dragons.