Spear of Pain is a tier 2 epic Two-Handing Stabbing weapon. It is preceded by the tier 1 epic, Frost Spike.
Void Core: 22
Clearly an offensively oriented weapon, the Spear of Pain suffers in that both its bonuses and its best wielder (the Amazon) derive their offensive power through critical hits. As a tier 2 weapon without game-changing bonuses, the Spear of Pain has lower priority relative to other T2 equipment. On top of this, the tier 2 plate armor, The Fortress, grants the "Fortified" bonus which makes one immune to critical hits. Since the Amazon can only attack the front line due to being purely melee, all of her unique offensive power from critical hits is utterly negated against any plate-wearing tanks.
The Cavalier can also use the Spear of Pain as a damage dealer, combining with the Dreadplate armour to give 65% bonus crit damage, although his long cooldowns make this risky. If it's even viable, it will require some finesse in team composition.
In PvE anything is viable, although even in tower attacks one generally doesn't lack for single-target damage, making the Spear of Pain currently of dubious application.