I'll agree that it requires the right team to have a proper chance to do it without possibly hindering your team in other areas. In my game, we had two solid front line / initiators plus two high-damage sources. I wasn't required to be that high-AP damage threat soraka that is required at times. And with the front line we had, I was able to stay safe and keep the mogs passive running for long enough in the fight for it to really make a difference.
But I still did reach CDR cap (I did have to upgrade my chalice to Athene's later on because I was out-surviving my mana pool). So I didn't feel like the amount of healing I was putting out was suffering that much. And I could cast heals on cooldown without a care in the world because my HP would just immediately tick back up.
Don't think it was straight-up OP. I'm sure it was frustrating to play against though (even if they wanted to pick off the healer to stop the heals, they couldn't). And I still believe that it even being a remotely valid option on a champion with a mechanic that's anti-HP stacking, it speaks to the strength of the item itself.