by Gewar
Yes - it is lure. You should play it when your opponent would except the other think (ie just at the begining of the game or just after resolving Nulify on your creature). You should not use it as a trap, if you never use one.Properly played Decoy either waste your oponent's Seeking Dispel or prevent him from cast the most powerfull spell.
Bonus :D
If you play it with Fellella, Pixie Familiar, you may use it to "transfer" mana from her to your mage (she pays 2 mana from her suplies and when Decoy is revealed/destroyed, you gain 2 mana).