Pros: Thanks to the high 0.9 AP ratio on Zilean's Q you will have high poke potential and will have very high mobility late game.
You will be able to get a lot of assists by using E on Assassins.

Cons: You will always be susceptible to large amount of damage and normally focused upon during early stages of team-fights.
If you are not careful in managing your mana as the precious resource it is (regardless of your build at any point in the game) you will find yourself in a very nasty predicament when faced with an enemy with high mobility.

Zilean is a very high damage champion who utilises Mana and Ability Power like no other. Caution must always be applied due to his very weak defense and low early mobility when auto-attacking.