There has been a lot of incorrect stuff written about these little gizmos, so having recieved mine I thought I would put the record straight.
Standard CAT5 network cabling uses an outer cable with 4 twisted pairs of wire inside. Although all 8 wires are terminated in patch panels, wall outlets, patch leads etc., only 4 wires (2 pairs) are used for ethernet. The other cables might be used if you are using the cabling infrastructure for telephone but in most instances they are redundant.
What these little friends do is allow you to utilise those spare pairs of cable to double the number of netowrk sockets on your network. You need one at the wall socket end where you have your PC etc and another at your patch panel/switch/router end. They work by connecting the normal pins 1,2,3 & 6 from your PC, printer etc to pins 1,2,3 & 6 on one of their sockets and do a cross-over to the unused pins 4,5,7 & 8 on the other so that you can utilise all 4 pairs in the cable. At the other end you use another one to connect the normal pins straight through again but cross back pins 4,5,7 & 8 onto pins 1,2,3 & 6.
And that's it. They do not provide sharing options or any of the other things people have written and have no impact on bandwidth or signal etc. because all they are doing is using otherwise redundant wires within the system.
They simply provide a quick, cheap, alternative to running a second, normally wired cable round your building and back to your patch panel by utilising spare cores within the existing wiring. This means every man and his dog can double the number of "wall outlets" on the network simply by plugging one of these things in at each end of the infrastructure wiring.
For 1 or 2 locations they are therefore fantastic. Whether using these in a commercial environment is ultimately cheaper than running more cable is for you to decide - lol.
I think they are great and just show that two into one does go!