It's extremely handy for Mages. Note that mana blocked hits do not increase Magic Level. While this spell is active, all damage received will be displayed in the Game console as Blue Text and the following icon will appear in your condition bar: .
The Magic Shield only absorbs a limited amount of damage before breaking. When this mana is depleted the spell effect disappears. The amount of mana it absorbs depends on the character's Level and Magic Level, including equipment and skill boosts active at the moment the spell is cast, according to this formula (rounded up to the next whole number):
Recasting the spell restores the shield to its maximum value. While the spell is on cooldown, mages can renew the shield by using a Magic Shield Potion. The Magic Shield can be removed by casting the Cancel Magic Shield spell.
Before the Vocation Adjustments Update, the Magic Shield had the same amount of mana the character currently had instead of a fraction of it. Furthermore, with this update the spell cooldown was increased from 2 to 14 seconds while its duration was reduced from 200 to 60 seconds. On the following week, with Update 12.60.10468, the Magic Shield duration was increased to 180 seconds.