I got Tivo. I think its awesome, almost 7 years old and still kickin.
Tivo has a monthly subscription charge or (?)$300(?) lifetime fee. You can plug it into your desktop and transfer tv show you want to keep which play on your ipod or psp. and if its hooked to the net you can stream podcasts and amazon ondemand movies straight to it.
Comcast- already has xfinity ondemand on it, I imagine you can transfer show to your desktop, but havent tried. and xfinity already streams anything youd already get on your ondemand box through their website. monthly service fee that adds to the alredy existing box rental.
*Tivo & comcast cable boxes do not work well with each other
Dont know much about google tv except that many carriers and distributors are denying them content (fox, comcast, etc)