I was testing out Darius who has one of the highest, the second highest I believe, health regen base stats at lvl 18. I had Warmogs, Spirit Visage, Titanic Hydra, Sterak's Gage, Black Cleaver, and Trinity Force as items, so +450% base HP regen and around 5k HP.
With Warmog's Heart active, I was regenerating over 200 HP a second and nearly 1100 HP every 5 seconds. It was so ridiculous that I was questioning whether Warmogs was bugged or not. Of course in an actual game, I wouldn't have my entire build complete, but with just Warmog's Heart, Spirit Visage, and some more HP and HP regen, you would still be regenerating like crazy.
Is this intentional? Warmog's Heart is almost twice as strong as Garen's passive at lvl 16.
I can see easy abuse from this. Vladimir, melee supports, HP tanks, would love this item. Maybe even an HP Katarina build... oh boy