It is cast as an immediate action - so if a creature charges a character they should be able to cast EFS to stop whatever horrid beast is baring down on them, or at least make them run into a forcewall.
But... what if they were casting already?
Wizard Bob's turn: He starts casting Summon Monster V
Hack'em the hill giant's turn: He charges Wizard Bob with an intent to demonstrate how he got his name.
Wizard Bob declares he wants to use an immediate action to cast Emergency Force Sphere.
Can Wizard Bob save himself?
Does casting Emergency Force Sphere affect his casting of Summon Monster V?