For cable modems the US cable companies do at least allow you to purchase your own box rather than rent one of theirs. There is a list of approved boxes, although in searching for the ones that support VoIP I found several hens' teeth on the journey. So now I have my own box, it's been installed just over a year and has probably just about paid for itself in terms of saved rental costs. It works quite well too, unlike the one they'd been renting me up to that point (which they'd broken with a firmware upgrade).
It would be interesting to see how this works out with the TV boxes, there's a whole different product requirement there.
On the other side of things, I don't remember having a choice of box with UK cable, but then there wasn't an explicit line item for rental either, and part of the sales pitch was that if it broke they'd fix it or replace it for free. That doesn't happen if it's your own box.
Biting the hand that feeds IT
