Ryze is an excellent candidate for use with the Frozen Heart, since the damage of his abilities is increased with his maximum mana. The added mana and cooldown reduction also gives him extra sustainability in lane and the armor reduces physical damage taken.
Warwick is also an good user of Frozen Heart, as the mana helps compensate for his mana troubles, the armor will make him much more durable and the aura will improve his already awesome dueling.
For its combine cost of 650 gold, Frozen Heart gives 150 mana, 30 armor, 10% cooldown reduction, and an attack speed reducing aura.
Frozen Heart helps counter DPS carries like Aatrox, Tryndamere, and ADCs in general, since they all rely on auto-attacks and the decreased attack speed will reduce their damage output.
Owners of Frozen Heart benefit greatly from Oppression mastery, always having 3% damage reduction from all enemy champions in 700 range.