Evasion grants a 75 (+12.5 per level) health shield. Lasts 5 sec."
This grants you exactly 10% of Illidan's base health + 10% of his scaling health on his E per level.
UPDATE 2/14: After the bugfix to this talent, this is, in my opinion, the winner of this tier 100% of the time. The continuous Es you get throughout a teamfight will help your survivability by a great margin, especially after Sixth Sense . The bug used to be that Auto-Attacks would destroy your shield while your Evasion was still active, even though the Auto-Attacks "Missed". With this no longer a problem, it is now a potent AA and Spell Damage shield.
Before you say "10% of Illidan's HP is nothing, useless talent", know that you pop this several times in a fight...and dozens of times in a lane. Over the course of a teamfight you may very well save yourself 30% HP (Not counting Sixth Sense mitigation), which out-performs other talents in the tier.