[{quoted}](name=Word of the Lord,realm=NA,application-id=3ErqAdtq,discussion-id=QzBOEqH6,comment-id=0000,timestamp=2016-07-31T22:16:35.797+0000)
The biggest problem is that it doesn't have the stats that doran's ring does. You don't get the bonus health and while getting bonus mana, you don't get any sort of mana sustain which most mages need in the early laning phase. Couple that with the fact that you typically don't want to leave lane early to get corrupting potion upgrade as it often gives a bad back timing and you can see why it is a situational start. Just my $.02
Sure, it is definitely situational, but I'm not sure the reasons you gave are quite as strong as you think.
The bonus health is offset by the +25% healing from potions.
Refillable Potion offers 2 charges of 125 health over 12 seconds.
125 x 1.25 = 156.25 health over 12 seconds
156.25 / 12 = 13.02 health per second.
Which means it takes 4.6 seconds to regenerate 60 health.
Effectively using potions during exchanges will lead to a better early laning phase, in my opinion.
You are correct that it does not give any mana regeneration but I honestly think the flat mana is more of an impact.
Doran's Ring gives +50% mana regeneration.
All champions have between 5 and 11.5 mp5 (regenerates x mana per 5 seconds) at level 1.
5 x 1.5 = 7.5 mp5
11.5 x 1.5 = 17.25 mp5
Which means it takes between 69 and 29 seconds to regenerate 100 mana respectively.
Now, that definitely isn't bad at all, especially when you keep in mind that this continues to scale up with level, but when upgrading to Corrupting Potion you gain 75 mana, and Touch of Corruption over 12 seconds, and an additional charge, and it refills every time you back.
In most cases, both, Doran's Ring and Corrupting Potion will be sold for the item slot.
Doran's Ring sells for 400 gold
Corrupting Potion sells for 350 gold.
Assuming you use all 3 charges every time you back, and assuming each charge is the equivalent cost of a single health potion (which it is actually more) that is 150 gold saved every back. This is already cost effective on your third back.
As far as when to go back and ultimately upgrading to Corrupting Potion, it may not be best to go back as soon as possible. Even if you were to not upgrade on your first time back, the bonus health from Dark Seal makes Refillable Potion cost effective on the first back.
Go to lane, then at 350 gold, go back and grab Corrupting Potion.
I'm actually not certain that Dark Seal increases the amount of health that you get from Corrupting Potion like it does with Refillable Potion, but regardless, this combination makes it so you can be very aggressive. You try to trade without taking damage of course, but inevitably you will end up taking some. Try and wait for a minion advantage then pop a charge and go in. You will almost certainly out trade your opponent either giving you the kill or forcing them out of the lane. If you get the kill, you get stacks of Glory, which means more AP, which means you are starting to snowball already.