Sorry, I'm going to have to cast my vote for "too powerful". I DM a group of three players, running evil characters, and we OK'd the Book of Vile Darkness for use. One of the weapon enhancements is "MarrowCrushing". +3 market value, deals 1 point of Con damage per hit, bludgeoning weapons only. It took me all of two combats to see how horribly, horribly wrong it was.
Here's the simplest reason why 1 point of Constitution damage per hit is too powerful:
Normal weapons deal a fixed amount of damage. Higher level characters gain iterative attacks to deal more damage, and higher CR monsters have more HD, which gives more HP, to take more damage. Weapon damage does not scale with level. It is a fixed constant.
Constitution damage, OTOH, does scale with HD, and thus with level & CR. Put another way, a 2 HD monster & an 18 HD monster that each take two points of Constitution damage suffer the exact same proportional loss of health.
Still not seeing my point? Look at other weapon enhancements: A flaming weapon deals +1d6 damage with every hit. That's 1 to 6 points, unless the creature has Resistances or immunities, which might reduce it, or vulnerabilities, which might increase it. Now compare that to Constitution damage, where every hit has a 50% chance to reduce the creature's total hp by one per HD. It might do no damage (odd Con score), but if it does do damage, it's not a fixed amount, but a fixed ratio based on the victim's HD. Hit a 10 HD creature, its hit point total drops by 10 and it takes the weapon damage. Hit a 20 HD creature, and you actually do more damage. That's right, you do more damage against tougher creatures. Go ahead & find me any other mechanic, anywhere in weapons, armor, spells, magic, or feats, where you will consistantly do more damage against tougher targets than you will against weaker ones. For that matter, find me a mechanic anywhere that has the character's die rolls being dependant on the target.
I didn't like this mechanic after I saw it playtested from the BoVD. I like it a lot less now that it's core.