Text (4th+errata): Exile target creature. Its controller gains life equal to its power. [Oracle 2009/07/08]
The controller of the creature may decide to "pump up" the creature before it leaves in order to get more life out of the deal because the total power of the creature (including enchantments and such) is counted on resolution of this spell. [D'Angelo 1994/04/01]
If the creature has a negative power, the player does not lose life. It acts the same as if it had a power of zero. [Aahz 1994/06/01] See