A mage using invisibility is somewhat like a rogues vanish or a hunters feign death.  Only not as effective as not getting killed in a wipe or getting away from mobs.  It takes 5 seconds for it to take effect.  If your in a 25 man raid and it wipes, you can throw this up and hopefully not get hit within the 5 seconds and when he resets you will be alive among hunters and rogues.  Or if you are PVEing you can frost nova whatever is about to kill you, blink away, and pop invisibility and it gets you out of combat.  They will run away and you can eat and drink.  Keep in mind that casting any spells or doing ANYTHING other than running or standing there will make you come out of invisibility.