I'm a big fan of this card. Put this on something like a Ferropede, and you're basically casting Thunderclap each turn. Put it on a bigger creature, the more damage you'll be dealing. It's really effective because your opponent either has to block the creature, thus freeing up a slot for another creature to get through, or allow you to destroy an important creature (like Immerwolf or Insectile Aberration). It's a little expensive, but it has the capability of doing a lot of damage. A funny thing to do with this card is put it on a Laccolith creature (like Laccolith Titan) and, no matter what your opponent does, a creature on the other side of the field will die. Now, a problem with this card (aside from cost) is that the creature needs to be unblocked for the effect to trigger, meaning that a Thrull can ruin your plans altogether.
Pros: White direct damage, repeatable creature removal
Cons: Requires that the creature gets through