This item needs to be revalued. It was updated in patch 3.0.8 and has a 1 minute cooldown and 10 seconds of uptime now(the tooltip is incorrect). which is a 100% increase in use time, and a 25% increase in uptime.
This is now 56.6666666 Haste over time, not 24.3.
If the iLvl of a gem is any indication of value, with 19 spellpower on a gem being equal to 16 haste, I believe this is 2.403 times the value of the 28 spellpower enchant, as it would be equal to 67 spellpower.
This also can be stacked with cooldowns, such as with my mage; My mana gem bonus for 7.5 set, combustion, icy veins all are on same macro with this so whenever my icy veins is up its like a huge bonus instead of a little bonus, coupled with the spellpower of the mana gem.