That item has to be the most brroken, unfair Item in the game right now. For it's cost, the amount of stats it gives is just unreal and absolute bullshit.
Remove CDR from it for heaven's sake and increase the CD of the active to 4 minutes instead of 5 seconds. It's bullshit having to deal with that item..
They balanced it by adding stopwatch to different items like guardian angel and Stoneplate. Also, its the only ap item that grants Armor. Whereas Maw of Malmortius and Mercurial semetar are two strong AP counter items for ad champs. They cant nerf something that is the only saving grace for AP champs that have to deal with AD assassins. If they want to nerf it, they would need another item that grants mages armor in order to compensate.
Also, its a 90 second cd. And most of the time, burst abilities are up before that cooldown anyway. Hell you can even get karthus ult lower than 90 seconds.
It used to give 120 ap and 70 armor. Trust me, it's been nerfed. It also had it's cd increased.
Ill agree that 
is the superior build path.
AP: 70 --> 90
Armor: 45 --> 55
CDR: 10% 0%
Cost increased by 300g (combine cost increased by 250g due to recipe change)
CD starts at the end of stasis, rather than on cast
Zhonyas now feels better in a late game build (after Rabadons passive, it provides 126AP instead of 98!) but costs slightly more to pay for those stats.
However, Transcendence builds take a slight hit compared to Absolute Focus users.
Capping CDR and having Rabadons + Transcendence + Gathering Storm with 3 stacks ... someone else do the math how the AP from JUST Zhonyas compares.
