Tired of getting killed? Tired of running out of mana?
Try this out and stop both.
You do sacrifice about 20% damage for about 200% more survivability and sustainability.
Keeps you in the fight and punishes those that try to focus you (which is every game).

Throw Ring of Frost down right on top of yourself, then Improved Ice Block, and watch them sit there and tank every ounce of damage your team can dish out.

Frost Armor REALLY throws a wrench into melee and ranged assassins gears. It is SIGNIFICANTLY more difficult to kill Jaina. Once they hit you they are slowed 35%, you take 75% less damage from the first hit and you simply walk away.