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In my experience, Heartseeker is more or less worthless early on, besides the crit passive. The cooldown is ridiculous, the mana cost is even more so considering how little mana you're going to have available, and the damage sucks. You're going to need to be spamming the spear throw, which means you really can't afford to drop 120 mana in a single burst that doesn't do that much unless you burnt another 70 mana on the stun.
Once you build up some attack power items and get a larger maximum mana pool, it starts looking much better. If you grab attack power early, Heartseeker can shred through enemies in the midgame.