I think that you can only devour certain enemies (Malboro, T-Rexaur, Cactuar etc.), but the rewards are quite good. It's not a command you'd want to have constantly, but use it when you go to the Islands closest to Heaven and Hell and you should be able to raise your stats a bit =)

Edit: I found this list on ffinsider.net of enemies you can devour:

Hp: Rinoa in Adel + 10 HP, Ruby Dragon + 10 HP
Str: T-Rexaur + 1 Str
Vit: Adamantoise + 1 Vit
Mag: Behemoth + 1 Mag
Spr: Malboro + 1 Spr
Spd: Pu Pu + 1 Spd
If you eat a Cactuar your health is fully replenished.