Where to start? These little gadgets allow you to send standard video signals down cat5 computer network cable and allow distances of up to 300+metres(!) without any signal degradation. Standard s-video cables, although dependant on cable quality, will only run 30-50m before experiencing signal loss. For all the detailed tech info check the link below to the balun FAQ - I'll just cover the VJ applications.... With a couple of these baluns you can easily turn ANY network cables to S-Video and allow for quick and easy cable switches. Run a cable to both sides of the stage give the camera man a balun to plugin and have both cables plugged into your mixer. Quick and easy swapping of positions.
Cat5 can be purchased for US25-50c a metre and with not a lot of experience can be constructed on the spot. We recently did an outdoor party and made all the cables on the spot and due to cat5's low coast ran a redundency for each cable made - with 3 zones all with 3 or 4 30m to 50m runs the expense of regular SVHS would have been near impossible for us.
Basically if you can afford the minimal outlay for a few of these I can't see why you wouldn't use these for all your cable runs over 5m.