I'm on Comcast and, like many others, realized I was wasting $7 a month on renting a cable modem. I bought this 'non-retail box' Motorola SB6121 for $76 and change. It arrived quickly as all my past Amazon purchases have. I unplugged the rental, pluged in this new modem, called Comcast to register it, and it worked. No issues. I've had it about a month now and have had perfect service.
I keep asking how Best Buy stays in business. I'm buying everything from Amazon these days.