Objectively...

With my i7 920, stock speed, stock cooler.
With 5-boxing, using IS Boxer.
With every physical core on all five instances of the game.
And no logical cores assigned to any wow instance.

I get up to 30% CPU usage in battlegrounds.
Dalaran will occasionally reach 50% or so.

CPU is not my bottleneck.
Nothing is really close to 100% usage anymore.

For the price, definitely not worth it.
But, then again, if you like to have the best.
And have the cash...
It is tempting.