They're mostly good for being in stealth, triple sparking and trying to OP roll people in PK.
He's completely wrong, only stupid sins do this. You can research demon/sage skills for each class here http://pwi.ecatomb.net/skill.php.
Unfortunately, most sins are played exactly in the way described. Stealth > Spark > Occult > Dead > Stealth. Good sins use their CoTD in combination with their skills in order to play a caster sin aka DPH sin. Those sins are able to do huge amounts of damage with their skills if lucky with their GoF procs on r8 daggers (that isnt to say APS sins are all terrible, there are some who know what they're doing, most do not). Sins are an excellent class in 1v1 situations and farm with, but when it comes to mass pk they lack usefulness other than focusing one person.
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Sin is only good for pk really and can be good for tw as well it all depends on how you use it. Any class is good if you know how to play it correctly. But the whole spark stealth thing that is beri noob.
I'd say sage>demon.. but you can make your own choice.They are useful for solo farming because they can recover their hp while attacking, which comes in handy when grinding down a ton of hp on bosses and aoeing mobs. That and, stealthing in instances. If you want to be rich by farming a sin is great to use.In pvp, well I wouldn't say they're useless in mass because in mass pk you have to take out a squad's priority targets (clerics, mystics, venos - all arcane, supportive classes). Therefore, while they can only focus on one person, they're able to drop them quickly, often without even being seen when they pop out of stealth/jump to the target.That being said they are squishy. At least sage spark gives damage reduction, but with all the attacks flying around they're at risk since they can't hang off to the side like archers.Ofc as others said, most sins (here, pwi, anywhere) suck. But you don't have to. It's just the easy way to play.