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I'm either looking at

Chaos Champ Undivided
Combat Blade & Power Sword & Power Armor & Camo Gear

Nurgle Marine, bolt gun, red dot, camo gear, combat blade and power armor
Nurgle Marine, bolt gun, red dot, camo gear, combat blade and power armor
Nurgle Marine, bolt gun, red dot, camo gear, combat blade and power armor

Cultist Improvised Armor, combat blade and auto gun
Cultist Improvised Armor, combat blade and auto gun
Cultist Improvised Armor, combat blade and auto gun

or

Troupe Master, fusion pistol, harlequin blade, photo visor
Virtuoso, neuro-disruptor, harlequin blade, photo visor
Virtuoso, neuro disruptor, harlequin blade, concealed blade

I like the chaos spam, but if its a bunch of useless guys, then I'd rather play cagey with the three harlequins that can mow through opponents who spread out enough for me to abuse them.

Played this weekend, after my first game I realized that Harlequins are fun and nasty. I wonder how I would feel if I had to play against them.

Sounds like you found your answer. I would like to add this, play with what you have fun with.

Harlequins sound like annoying gits to play against from most people I've heard from...which sounds perfectly in character for any Eldar force

I'm curious to go up against them as well and see what weaknesses could be exploited. Certainly taken them on in a straight fight won't work...since they won't get into a straight up fisticuffs if they're smart.

The effectiveness mostly depends on terrain. Harlequins will have an advantage on densely packed boards but if they won't be able to hide out of los or at least get decent cover, they'll go down.
It's a good idea to get chaos gunners. They're often more effective than regular dudes for points. Even if you don't want to spend points on AC or HB - both amazing but expensive weapons - you can always get a cheaper plasma or flamer.

Oh, also, don't even try fighting a solitaire in mellee. Especially if you don't get advances.

Harlequins suck in some missions and light terrain, but otherwise are really powerful. I would say run some practice matches with both. (Not like you really need the models if you're playing with cool cats.)