This little box has done the trick for me, boosting the "Poor" or "Very poor" signal from a BT Home Hub to "Good" or "Very good" in my top (second) floor office.
It's not plug and play, but installation is fairly straightforward if you follow the instructions carefully. The manual omits one important point: You'll almost certainly end up changing the IP address of the WL-130 to work with your own network router. You should make a note of the new address so you can log on to the WL-130 to make changes in the future. I bought mine from a different supplier and I think the unit had been tried out by someone else, so the IP address was not the default one given in the manual; I had to press the reset button a couple of times to get the factory defaults back so that I could log in to the box and configure it for my setup.
Also, try out various locations and aerial orientations to get the best signal - the most obvious position may not be the best. Mine came with a bracket you can screw to the wall for permanent fixing. The cable on the power supply is a little short, but I'm sure you can buy a replacement if this is an issue for you.