tl;dr Just because you run out of mana doesn't mean you should get it. This item was made for the actual mana hungry adc's ie Varus, Trist, and Sivir. Stop forcing it into your build.
I'm not talking about the people this item was meant to help (Jayce, Poppy, Yorick, mana hungry aa dependent champs in general). I'm talking every ranged adc taking Essence Reaver into bot lane as a first item, let alone them building it at all.
It's 2650 gold that is taken away from building a Last Whisper. Building it as your sustain weapon leaves you stuck laning against someone with BT or BotRK which have more kill potential in my honest opinion.
I can understand buying it if you are behind and plan to do the farming game on some adc's, but I still feel early game you should be striving for better items. BotRK is still really good, if anything the damage got a nice buff and makes it even better for dueling. Blood Thirster has a good 30 ad on ER, but understandably you won't be putting down 3.5 k gold before the enemy laner has an Essence Reaver theoretically.
Sure getting mana back per auto-attack is nice, but if Lucian got by just fine without Mana Steal (that just sounds lame) before, then why waste potential burst on an item as weak as ER?
There are a few exceptions I'd add to a list of adc's it would be good on, it wouldn't be anyone who was in the meta prior to the new adc changes. Stop taking ER on Lucian and Draven; Varus, Sivir and Trist can get more use out of it. --- Liquid Nitrogen is dangerous because it can freeze anything in seconds, which begs the question... Will it blend?
The way i see it, Reaver works best on champions like Varus who do most of their damage through poking with mana intensive spells. Varus sits back, throws Q's out then sustains the mana back and does it again until eventually you're almost dead and then he goes for the kill.
People i see it working on.
Varus. Sivir. A Cait who has really good aim with Q. (Not optimal on her ever though.) Corki. Ezreal. Maybe, maybe Ashe (IE rush is probably better though).
Mana sustain is important only if you're using that mana to consistently poke people. In a straight out fight between a Reaver+Pick vs a person with BT, BT should win if they're equal. However if your pick can consistently poke from a range where it's hard for them to retaliate from, then the mana regeneration becomes useful.
Basically you need the following traits to optimize it.
Good poking. (Varus Q, Sivir W and Q, Ezreal everything, Corki Q, Koggy everything, etc) Mana intensive when poking. Weak(ish) all in. High ability usage over AA reliant (Sivir instead of Vayne as an example).
If you don't fit those traits, don't buy it. --- I laugh as I watch people ignore the past, lie about the present, and misrepresent the future.
Essence reaver + Trinity force costs right around the same as Bloodthirster + Phantom dancer
it was made for certain adc's (Ezreal notably) and it works well with several adc's, notably sivir, draven (infinite W is always nice), MF (with TF, it's great) varus, lucian (though it's situational on him), corki, trist (JK, she doesn't need this, she should be getting botrk) --- "The good thing about science is that it's true whether or not you believe in it." - Neil DeGrasse Tyson
I didn't think of MF. MF also fits most of the traits i listed.
Draven is better served with IE or BT first just because of how his Q works. You really shouldn't be using W so often in lane that you're oom because of it. --- I laugh as I watch people ignore the past, lie about the present, and misrepresent the future.
Lucian got by just fine without Mana Steal (that just sounds lame)
It also doesn't make much sense. Life steal is you stealing life. You aren't stealing any mana from them, their mana is untouched. The stat is Life Stolen Converted to Mana, which sounds much cooler. --- Press 'X' to receive your interactive, cinematic experience.