Going down the road with Comcast I was facing $7.00 per month to rent a modem.
I could buy my own. I came across a good price on a Cisco DPC3008 DOCSIS 3.0 modem and grabbed it.
The Comcast installer didn't blink an eye about it and installed it and activated it with no questions asked.
Interestingly it looks like Comcast downloaded their own firmware onto it.
And it's fast.