With all the changes to Armor pen preseason, Warmog's has become my go to defensive item.
First off, it's REALLY REALLY EASY to get high armor pen now, compared to S2. I mean, if you look at the numbers, and play a few games, you'll realize that people can get S2 levels of late game armor pen in mid game S3, which massively buffs all AD champs. It's no longer ok to build pieces of late game armor items, and save up for expensive stuff. Now you have to build something, or they will be dealing true damage if they have any clue how to build. On the other hand, if you just build warmogs, not only do you get some great defensive ability with the massive hp boost, but you also grab sustain as well.
Second off, the fact that Warmog's doesn't have to charge up anymore really helps. Before, you had to worry about killing minions to get the full effect of warmogs, which made you take risks in order to get those cs. Now you can just buy and and run into battle, one of your own choosing rather than getting ganked while trying to farm up warmogs.
Finally, the regeneration is the tits. It's not like it's a lot, but it's "significant", which means that you can tell it's there. The fact of the matter is, sustainability is a big issue for a lot of champs, and during mid game where there's a lot of roaming around and ganking, there's really no time to go back, and pots don't realy do that much. At times like this, it's better to have warmogs
It's not like warmogs got buffs, but, in this game, it warmogs just got comparatively greater.
To those of you who say: But wait: Warmog's is countered by % dmg!
I say to you this: Sure, if you are willing to trade that BT for a Blade of the Ruined King. There simply isn't room to itemize for % dmg in today's meta.