Reworked Zilean is one of my top 3 played champions in ranked standing on a 65% winrate
http://matchhistory.na.leagueoflegends.com/en/#match-history/NA1/214137494?champion=26&matchType=2
I consider myself a post-rework zilean main, and I can honestly say he's really bad right now, BUT not as bad as people make him out to be. He's actually a decent mid laner and an extremely safe pick. Decent poke dmg once you get the ap (albeit unreliable), speed + slow makes dealing with ganks from allies or enemies easy, and you have your ult to fall back on. He's actually a much better mid laner than he is a support. Mid lane zilean has serious potential, and we shouldn't over buff him just because he's hard to play. These are the changes I think would put him in the right spot:
Passive is absolute garbage. They gut a global passive to just your lane partner, without any compensation at all. Either increase the experience gain to something useful or give him a new passive. I'd say its one the worst, if not the worst, passives in the game right now. It doesn't even do anything at level 18. It's just not good. Seriously, improving Zilean's passive is probably the easiest change you could give him. You don't even have to think about it. Just do it.
Bombs are simply too hard to land. They need to be skillshots, no doubt, but I don't think going from point and click to almost impossible to land on anyone once they get tier 2 boots was a good idea. Reduce the cast time or the time it takes to land or something. And I know someone's going to say "b-but the stun is so good", it's really not all that. That whole "land 2 Qs and stun entire enemy team" is just a fairy tale. It doesn't actually happen, and in the small chance you land 2Qs in a teamfight, the blast radius is only big enough to stun 2, maybe 3 if your lucky, people for 1.5 seconds. Just make them easier to land and problem solved.
W is fine imo.
Just revert the E change completely. Movement speed cap doesn't make it much better than 55%, the speed/ slow was way better before with a longer duration. I don't even see the reason for the change. Just change it back completely.
I'm honestly fine with the ult nerf. I didn't like having to constantly spam w anyway.
I'd say mid lane zilean is ok, but support zilean is absolutely awful. Jack of no trades and definitely not the master of any. Unreliable poke with non existent damage unless with a significant amount of ap, which is hard to achieve while supporting. Stun is extremely hard to utilize. No hard engage. No sustain. Pretty much have to build a mana item to be useful, and have to build ap to do a sliver of damage in lane. Also gets blown up if you don't ult yourself. Just so, so so so bad. And to makes matters even worse, a good portion of supports have shields and heals to block his bombs because they're delayed. Not only that, but adcs can easily dodge them with dashes.
TL;DR Zilean mid is ok but not as bad as people think. Passive is terrible and ineffective. Make bombs easier to land. Revert E change. Support zilean is awful.