This item was debuffed in the 1.12.1 patch. It used to increase casting speed by 25%.
Per Casting Speed calculations, this item will increase spell casting speed by .2102, or 17.4%. A 3.5 second spell will be cast in 2.89 seconds.
This item grants:
330 * (20 / 300) = 22 Haste Rating
Or in other words 1.395% Spell Haste.