i tried it in a few games vs. CPU today just to see if it really does have a noticeable impact.
i found that it does not. slowing auto-attacks by 30% for the first few seconds of a battle is going to save you somewhere in the range of about 200-300 damage (depending on armor levels, enemy attack speed and damage, being in melee range, etc.)
its really a very small amount and not all enemy DG's will actually care that much. doesn't effect their minions or damage spells so you may not even be debuffing the most relevant damage source on the field.
and yea, it bugged out on me alot. got the "stuck for 10 seconds" bug on several occasions. that bug alone is such a huge risk that i would never use this item in a competitive match.