On the linked page, it talks about getting ghosting or smearing with an 800x600 display over 50 feet of cable. This is definitely a try-it-and-see kind of application.
There is nothing special about this. You can use any wire to extend a vga signal. Pick up some Romex or extension chords at HomeDepot and have at it - all it's got to do is conduct electricity. Just about any wire you use will cause image degradation just because of the length. I don't think there are any real properties in cat5 that make it work much better than anything else - the twist is probably not sufficient and its effects may vary greatly depending on which lines are twisted together.