While I like playing Zil, I think the AP ratio on his bombs might be a little high. I almost got a 1 shot kill with a double bomb mid-late game, and I wasn't very fed...

Aye, but once you used that double bomb, you can't use it again for 15 seconds isn't it?
And compared to Veigar, whose Baleful Strike can increase his AP, he gets hitting hard mid game as well, even harder than Zil, who only has that 1 damage spell.
The Bombs end up becoming little more than a pain end game unless you've gotten 20 kill...ish (any hero is bad at 20 kills, Zil though... ugggg, no one can match the damage... so much death.)
Of course, this makes Zilean all the way through the game, in the beginning-mid portion, he's a great harasser, from mid-end, he's a wonderful support, his ult has an ap ratio of 1:3 ain't it? So he can end up rezzing the tank to almost full HP if he stacks AP (which I've noticed most other Zils do, and I don't blame em.)
Personally I go Mana/Cooldown/AP and hope for the best, it seems to work if my teammates know what the three circling sand timers mean...
Always worse when you have to explain it more than once...

The AP ratio I believe to be perfectly fine. People need to remember that Zilean is also classified as a "Nuker" and this is only possible with the current AP ratio. He's not an assassin or a fighter. Why? Because he only has one ability that can actually deal damage. There is no flow to his damage, it is not consistent. It's a real quick combo (Time Bomb>Rewind>Time Bomb) that accomplishes the burst damage. That is a Nuker. That is the reason why you see Zileans use hit and run tactics.

My only real complaint is his raw ability to harass early game. Which is due to 2 parts.

First, *placing* his bomb doesn't draw aggro (from creeps or towers). This allows him to tower dive for one hit without the tower taking any shots back at him, as well as harass you behind your creeps without drawing any creep damage.

The second part is simply the range:cooldown:damage combination. It is a never-miss attack (0 wasted mana), that can do extra damage if you don't run out all alone somewhere to blow up. In spite of that, it has both high damage and low-ish cooldown. 15 seconds is not much cd for a harass ability, especially if you're "pro" and laning with someone who can aid your mana regeneration (mana manip or clarity).

The two changes that I'd make to Zilean (that won't actually change his power level) are making CASTING the bomb cause aggro (thus drawing creep/tower fire immediately), and raising the cooldown at low ranks from 15 to 23 seconds (-2 second cd per rank). This cd scheme is what Ashe currently deals with (or volley would be seriously OP early game, without the initial 20 second cooldown).

^ Viro speaks the truth

The only real problem I have with Zilean is his early game abilities. He farms and farms and farms and farms, he won't get hit. If you get close he'll just run away. Now now, I'm not saying he's OP or too strong, not at all. I am just stating he's really good early game.

The last game I played versus a Zilean was fun to play, but he gimped all my kills. I ended with 2-7. He stopped me from really ending someone with his skill, I think he derived me of at least 12 kills. ^^

Now the real trouble was, as I said, early game. Zilean is 'the' anti-towerhugger. Due two lagouts (on both sides) I ended up Zilean vs Rammus. I lacked the ability to really kill him after he popped his first cat-life.

Twas a fun game, but his early game harassing is strong. His bombs end game were just tickles. Zil is fine.