Attack does 10% more damage and drains your opponent's combat statistics until they reach 0. Statistics are drained in the following order: Defence, Strength, Prayer, Attack, Magic, Ranged.
Now, I don't know a whole lot about pking, but I know pures tend to have low def and prayer. Which means Strength would be the first to go, followed by attack.
I haven't heard how fast the spec drains your combat stats. But it has to be pretty good to make up for the 15% strength difference between the bandos/armadyl specs.
The other thing to realize is that it's not quite as useful against someone who's not a pure, because it will drain defense before strength.
I don't exactly go out and kill pures for sport, but I think i just might be on to something. At this point, the sword is too expensive to see a whole lot of action, so I guess the thing to do is see what happens when they come down.