ZZ'rot is alot of fun but it is very gimmicky. Like the Blitzcrank of Items it has one trick and it is heavily dependent on the champ and situation.
If you are building it for the armor and MR than there are definitely better items with more beneficial passives and actives for the tank on the go. In fact i wouldnt even consider ZZ'rot for a tank build seeing as the stats are negligible and the active (void Gate) at best can just distract an opposing team for a few seconds. Largely due to the fact voidlings feel the need to fight everything in their way.
So unless you have a champ who can sit there on lane and babysit the portal (which is asking for salt if you are the team tank) it really is a useless active for the high cost.
ENTER TROLL PICK!
that being said I've found ZZ'Rot is best used on one and only one champion... A champion that screams troll from the select menu. A champion that has the time and ability to baby sit a lane with ease. And can passively clear the waves of minions so the little zerg spawns can direct their void fury on the turrets they hate so much.

I find that if you grab ZZ'rot portal as his second pick after zhonya's Heimy can camp a lane like a boyscout troop. drop his turrets in the overlapping triune pattern then drop that zzrot in the middle. Hiemy becomes a lane pushing monster.
No one else has the ability to baby sit the portal to maximize its full potential. Likewise the ZZ'rot gives Hiemy added defenses before he starts building deep AP.
Note: This build only works in soloque lower ELOs hiemy as a champ doesn't really bring much cohesion to a team fight which is the bread and butter of higher elos. However it is really fun to play and will bring alot of hate from the other team.