Essence Reaver has two main flaws. It's a dead end item, and it's build path makes no sense
Let's start with the first point being a dead end item.
Quite frankly, it suffers the same problem as old Malady, Haunted Guise, etc. The item is nice and cost efficient, but cost efficiency only goes so far, the item isn't slot efficient. You'd rather sell it and buy a Youmu's or something instead.
Solution: Up the item's price and make it stronger OR make it upgrade into something relevant? Like I don't know, if it upgrades, have it's mana drain passive extend to ABILITY damage as well as AA damage or something. Then maybe it might be useful.
As for the second point about it's build path making no damn sense. The ONLY champion who'd consider this item useful are super mana hungry AD champions. But mana for all the ADC's that'd want this item ceases to be a thing by the time you save up for it. The item has a mid game price, while it tries to solve an early game problem. What's worse, at no point in building this item are you getting any mana-aid that a champion that'd want this item really needs. If you're desperate enough to buy such a terrible item, it means you really need help with this item. But it's build path does NOTHING to help you sustain your mana!
Solution: Add a mana-related item in this item's recipe.
Those are the main issues. Asides from that, it appears to be a laning item. The mana regeneration passive will rarely come up much in a fight. Maybe it'd work out in a siege, but you'll only get so much mana back competing with your allies to kill the minions asap.