My aunt and uncle have one that they use when they're in FL for the winter at their home in canada, so I got a little exeprience with it when I was down there.
They seem to love it. The picture quality obviously isn't incredible, but it was more than watchable and probably wouldn't look to bad shown on a TV or Monitor from the computer/laptop.
Nick, unless there is a separate box you can buy for that scenario, the slingbox has to be hooked up through your cable box, it does not decipher and change the channels itself, rather it just commands the cable box adn then converts the A/V signal it gets from your box to be sent over the internet.
You could split the signal and have the tv connected to the cable without a cable box and change the channels with the tv or get another cable box.