Mark of Subversion?

For 4-9 seconds, the next time target foe casts a spell on an ally, the spell fails and you steal 10-76 health from that foe.
Now, when it says "an ally", does it mean one of your allies or one of your target's allies? In other words, would this skill be best spent casting it on an elementist, or on a monk?
Also, does anyone find this skill particularly useful? I mean, it sounds good, but it lasts for only nine seconds. Not exactly hard to wait out. Anyone find a good use for it?

Target's allies. Works well to counter healing, gives you a boost at the same time. It even affects spells the target casts on himself; thus it is most useful vs a monk really, as it counters a heal on him and deals a pile of damage, allowing a spike to go off more successfully.