They are very good. Ability Focus (hexblade curse) is good, very good, and Fortitude will help bring your Mettle into play. It's important to realize they are fighters first and casters second. Using their defense against ranged attacks and mirror image, they can be very effective as stand up fighters. At much higher levels, they qualify for Arcane Strike. With a ring of wizardry, they could be adding three or four dice of damage to nearly every attack. Improved Familiar, for a winter wolf, is nice, and I personally favor a glaive with Combat Reflexes and Hold the Line for armament.
For the most part, they work best as single-class characters. However, a three level Blackguard dip gets them aura of despair, which synergizes well with their curse ability, and a Havoc Mage can eventually cast every spell they have in melee without drawing an AoO.
Against no or low SR foes, their offensive spells can be nasty, and for everything else, they should focus on buffs, and try to hit enemy spellcasters with touch of idiocy.