I just grabbed it today (money isn't really a hard thing to come across so I have no problem trying experimental enchants). I know there is some hard core math out there that shows what I want to hear and if anyone could link that I'd be happy. Here however is what I've seen from this enchant from fighting in pvp today:
1.) It owns cloth/leather classes. You see nearly a 15% dps increase against cloth from this one enchant.
2.) It seems to proc often. However, I heard this proc isn't affected by your weapon speed or haste rating and that its a PPM.
3.) It is horrible against mail/plate classes, sporting at best a 2% increase in damage. Even with expose armor tossed up against your arena geared 70 warrior, he still has close to 11k armor and you're only ignoring ~3k of it. Since the armor scaling isn't linear, this works out to a rather small dps increase.
4.) The proc can't apply on both weapons (although we've known this one).
So... Looking more closely:
The enchant is ineffective against higher armored targets, isn't affected by weapon speed and can't be applied twice. The enchant is extremely effective against lower armoored targets however.
Taking all of this into consideration:
Executioner is an enchant specially designed for Warriors to hurt cloth/leather classes even more than what they already do. With executioner and the armor penetration from the PVP gear, it is quite possible for one warrior to get enough armoro penetration to completely ignore anyone not in mail's armor, thus greatly increasing their DPS. On War vs War the enchant is somewhat ineffective, but there it's really a matter of who is geared better than who.
From my tinkering around, I can tell that when I get a weapon I actually want to seriously pvp with (the season 3 maces for instance) I believe I'm going to enchant dual Mongoose. Seems executioner was just another gift to Warriors (a class that REALLY doesn't need any more gifts) after all.