this is info i read from japanese site. it increases ur critical rate depends on how manny tonberries killed with latent effect damage taken +10%.there peeps tested with 1200+ kill on tonberries and raised 5% critical rate if u can read japanese use this link http://wiki.ffo.jp/html/24028.html
Takes exactly 250 kills to cap hate if you start at 0. Or, just take the damage from Everyone's Grudge (without Shell/Magic DEF Bonus) and divide it by 5. Then subtract that number from 250 and that's how many more you need to kill for "capped" hate.
More than the Damage Taken+10%, I think the biggest drawback of this collar is how embarrassing it is helping a noob through the Temple of Uggalepih, only to get one-shotted by a Level40mob that's sitting at 3% Hp.
Other than that though, an unrivaled-by-slot enhancement. Get them by any means.
So far I have come to the following conclusions about this neckpiece:
1) Does not improve Everyone's Grudge damage (damage was static with no neck/Rancor collar on level 0 mobs)
2) Does not grant a new spell/ability/WS (tried on both jobs not native to tonberry and jobs that were)
3) Does not adjust visual statistics (Main stats, attack/defense/skill numbers/HP&MP etc)
I have not seen if it prevents accumulation of hate (all tests done at 2500 gil worth of hate) and Tonberries I did attack did not use Everyone's Grudge (maybe part of the effect? maybe not)
It took me about 240 kills (1,200 damage Everyone's Rancor) to get to the 6K Gil Reset cap, just as milestone for people to achieve. Galka if, for some strange reason, race is a relevant point of hate, heh.