You do realize that the vast majority of Cat5/5e/6 cable is UTP, meaning unshielded twisted pair, right?
There is shielded Cat5/5e/6 cable, but it's pricey.
That said, I have used Cat5/6 to extend things lke USB ports, LEDs & switches for the PC and the like. It's great for low-voltage stuff where EMI isn't that big an issue.
Some claim that it's good for audio signal, as well. I can't answer to that.