Black Cleaver is for AD and Zhonyas is for AP right? Or am I just all jumbled up in the brain... could be both though.
I've seen Zhonyas active used to good effect by someone people were they can make an escape with it but I've never been able to do it. I end up just lasting 2.5 more seconds and then dying anyway. xD
You normally use it to dodge something that would've killed you or put you in an extreme disposition.
That or if you're the assassin/mage that needs 2.5 or less for their next cooldown to survive/kill during a teamfight.
Wahooo