His jungle sustain is hilarious. That, combined with his manaless kit and AOE damage, you don't have to recall for sooooo long.
Technically he's in contention for 'strong AP tank' along with Amumu and Sejuani. Zac has advantages in higher mobility (and epic slingshot ganks) and disruption with his ult. He doesn't really have lockdown though, which the former two do have. His DPS is also a bit inconsistent unless you're constantly picking up your passive chunks, since grabbing them lowers his W CD so you can spam it.
So really, play him because you want to. He's very fun, and pulling off a long-ranged slingshot gank feels VERY satisfying. Also, his personality is genuinely awesome and hearing his quotes makes you feel better during a game, but hey, that might just be me.