Number of phones doesn't matter, it'll work on all of them. As long as you can daisychain the box between the phonepoint and the base DECT unit, and reach the USB slot on the PC at the same time, you're flying, otherwise it's not for you.
Advantages are that you're not tied to the PC during calls, you get the same quality of speaker/mic as you do with a regular call, SkyPe calls result in your regular phone/s ringing, and it's one less piece of junk to lose behind the sofa/sit on/break.
Noticed the same box for sale in the window of a (cheap) London retailer (Morgan Computers on New Oxford St.) over the weekend - at twice the price, so that 7dayshop price is genuinely good.