How would it negate Knock off? Why would that even matter? If someone used knock off on your pokemon holding a Mental Herb, it would be knocked off and... well... thats that.
Even if it DID "negate" Knock Off, they would use Knock Off, and the Mental Herb would be used, and Knock Off would have succeeded in... well... getting rid of the item.
wow. I could put this on shuckle and not worry about it being taunted for one move. But then, the opponent could use Taunt again when the item has been used. I'd say its not good, unless you needed that one extra turn.
So instead of suicide SR leads we get to deal with bulky SR leads holding Mental Herb this time? Great.
Or a Shell Smash Mental Herb sweeper. Heh.
This looks like a good item for pokemon that set up/dont attack. What pokemon could use this well?