Right now your almost always better off buying a bloodthirster instead of BTRK. BT is only 100g less and provides more AD and life steal. The current BTRK is horribly inefficient. ADC are better off with the increased life steal and AD for crits and AD casters and Bruisers are better off with the increased AD for damage. So i propose we change the passive from this

Unique: Your attacks deal 4% of the target's current health in physical damage and heals you for half the amount (120 max vs minions).

to this

Unique: Any non-periodic physical damage deals and additional 4% of the target's current health in physical damage and heals you for half the amount (120 max vs minions).

This would make the item more worth while on AD casters as it could supplement the need for an AD spell vamp item. It would also help increase the burst of AD casters who have multiple instances of damage in their combos( Talon,Zed,Pantheon ect..)

If needed we could reduce the effect for AOE skills.

You're ignoring the active effect on BotRK.

are you totally ignoring the fact that it steals your opponent's movement speed and gives it to yourself so not only are you slowing them, your speeding yourself up making it easier to stick to your enemy.

also lets do some math

a champion with 2000 hp, with bloodhirster you'll have no difference it still a 70-100 ad item with 12-18% life steal

with BotRK, your doing 40 AD + 4% of the target's hp (80) and your regen 10% +40 hp, so at this point of health, BotRK is adding more damage than your BT, by 20 AD, and for your BT to gain more health you'd have to be doing at minimum 2000 - 500 damage per hit. (the difference between 2-8% to get 40 hp)

so with these calculations we can say that if the champion is currently at 2000 hp your doing more damage and gaining more life back with BotRK than you would with BT, though if you had crits, this would be different because a crit with BT could potentially do more damage and have more life steal since BotRK's procs are not affected by crits. if the target had more hp such as a bruiser or tank, you'll see higher numbers and thus makes BotRK even better.

now if the target was at 1000 hp, then here are the numbers for BotRK, 40 AD + 4% (40) and regen 10% + 20 hp which would require BT user to do at least 1000-250 damage per attack.

so even with a champion with about 1000 hp BotRK will do better than a fresh BT but will do worst than a fully stack BT.

question becomes whats more important, getting that target down to 1000hp or finishing them off when they are at 1000hp?

if your the initator trying to do as much damage to a full health target, BotRK is the item to choose, if your a clean up crew and only attacking injured targets that are below 1000hp then BT is a better choice for you. and dont forget I didnt even take the active into consideration into these equations, which is an additional source of damage and healing.

well I just showed you the math, the values, and yes with a target less than 1500 BT would be better, but anything above that, BotRK will be better, plus you get the utility of the active as well.

for AD caster, your right, BT would be a better choice, though either way your wasting your life steal stat since if the abilities cant proc on hit effects, your not gaining from the lifesteal at all and just have an item slot for just 100 AD with nothing else, probably more beneifical to get something like a black cleaver which will give you reduction and health, and cooldown reduction.

so in conclusion, I suggest getting BT if your a clean up type champion who comes in after the fight has started, and getting BotRK if your part of the initiation and are an auto attacker.

It's worth mentioning it synergises with hurricane really well.
/shrug.

It's not an ad caster item (not enough AD) or an ad carry item (doesn't scale with crit), it's a damage items for bruisers to counter hp stacking (if you're playing Lee Sin vs Olaf, or if you are playing Irelia and the other team gets 3 warmog's). The problem is that Black Cleaver is such a better damage item for bruisers that nobody builds BotRK.