More people should play Zilean.
Seriously, his bombs take a bit of getting used to, but he is well worth it.
45% CDR can be reached really quickly with just

With that much CDR, you can bomb every 4.4 seconds and rewind every 3.3 seconds, meaning that you can doublebomb for a 1.5 second stun and 600 (+1.8 AP) magic damage every 4.4 seconds and speed/slow people ridiculously often.
Oh, and his ult is on a 33 second cooldown.
Is he OP? No, but he has enough damage, mobility, and utility that a skilled Zilean can make terrifying plays.
On a side note, Zilean is just stupidly powerful on Twisted Treeline with two bruisers, especially
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kit, the perma slow, useful ult when used in time, i would play him supp (or even mid) everytimes BUT... I JUST CAN'T STAND HIS OUTDATED 2009 MODEL AND VO, ITS ABOVE ME, I CAN'T PLAY VISUALLY UGLY CHAMPS... I CANT