Also it blocks what ever PCI-e slot you place it in so you'd need a board with an empty PCI-e slot (which kind of defeats the purpose of adding the extra power since now you have one fewer slots that needs power !!) and really if your using a high quality MOBO to begin with there should already be good enough traces on the MOBO to provide a steady power source to the PCI-e bus without the need for such a device - it might provide some stability for cheaply made MOBOs but you'd still be better off investing in a quality board to start with !
Also it blocks what ever PCI-e slot you place it in...
Unless you use the anyway useless PCI-1X / 4X slot.