I can't think of a single situation where you'd want to get Essence Reaver first. None.
AD champions tend to fall into one of two categories:
-ADCs (on whom you almost ALWAYS want to start IE, and only on a select few start Triforce)
-Melee AD (on whom you generally prefer Brutalizer items or Hydra, depending).
Because ADCs want damage at the expense of literally everything else, any item you get on them will probably have at least two of AD, AS, crit, and armor pen. They mainly rely on autoattacks as a rule, so Essence Reaver's CDR and mana restore tend to be somewhat wasted; if you want the 80AD and lifesteal, Bloodthirster is a flat-out better idea. An ADC without mana is still useful because autoattacks don't cost a thing.
Melee AD generally means bruisers and assassins. Assassins don't tend to autoattack, so the mana-on-auto is wasted; better to get the added burst of Tiamat's active, or Youmuu's/BC, or something else that will increase the damage you do over that crucial two-second-or-so attack window.
Bruisers, on the other hand, tend to mainly want stuff that will keep them in the fight longer (ex. resistances, health, Hydra lifesteal) or improve their effectiveness the longer they stay in (ex. BC, sustain damage). It's just too hard to justify an Essence Reaver purchase over, say, Maw of Malmortius, or Mercurial Scimitar, or (again) Bloodthirster, if you're sure you even want a big AD item.
As for who gets Essence Reaver...
-I've heard
has some success with it, but I don't play him enough to understand why.
-It's sometimes nice on
because it procs off his Q and gives him CDR (which he loves), but buying it early delays the damage Triforce/crit would give you, and by lategame you don't tend to care as much about mana anyway.
That's all I can think of.
I'd say you SHOULDN'T get ER on Kalista - Rend restores mana on kill anyway, so just make sure you have stacks on a minion as well as the lane opponent. Runaans has so much more value on her.
The biggest problem with Essence Reaver in my mind is that it's too niche for an early purchase, and its passive doesn't matter lategame. If I'm having mana issues in lane, I'm probably using a champion who likes Sheen/Triforce, so I go for that instead. If I'm playing a champion who won't get much out of Sheen, chances are I'm not casting enough spells for mana to be a problem.
It's a problem I could see solved by changing its build path (give it a "baby" version of the item that you buy for the passive, and then... I dunno, make it build out of Brutalizer?) but for now it's just a luxury item or a very, very edge-case item.