The Thermaltake Power Express 250W is an ingenious little device that has been realized in a very solid way. Since it goes into a 5.25" drive bay it will fit into all cases without the need for any modifications. The power-sense circuit makes the use easier because it detects when the power button is pressed, so the unit will only be running while your system is running. This also has the advantage that you won't have to press a second power button to start your system.
As our testing shows the Power Express can easily take a lot of load off your main power supply. Unfortunately in our testing we didn't see any improved CPU or video card overclocking from running with a second power supply. On the other hand the voltages were much more stable, which could mean higher overclocks in some cases.
This unit works best when paired with a cheaper power supply. Current high-end PSUs were already designed to supply insane amounts of power to the video cards, so there may be no benefits from going dual-PSU in such cases. But you have to consider that such a PSU can cost you well over $150. The Power Express sounds like a good way to skip a PSU upgrade when upgrading your system because you can continue using your current power supply.