Definitely not 50% maybe about 30% on EP/TW mobs, in exp on IT/VT mobs it was not uncommon for this thing to not proc once over the course of a 2hr exp party for me.
If you're a cheap PLD, use this. Get it when you can because it's a great sword. Only swords that outdamage this are Kaskara (if you want the extra VIT, but VIT is broken 70+), Company Sword (the standard that most PLDs use), Hrotti (if you're lucky enough to acquire this, never use anything else unless Macuahuitl +1), Hauteclaire, and Ex. It's a great sword.
Well, I obtained this sword enroute to the Savage Blade WSNM for my 74 PLD (I know, getting that WS way late, but I wanted to duo it). I decided to try it vs. the WSNM mob, Girtablublu, a high-level scorpion. Checking the log aftewards, it would appear the additional effect on this weapon didn't proc a single time. Not much help vs. a mob that deadly/dangerous, I'm afraid. I did much better vs. it when I switched to Durandal.