said by BigPappyJapp:
If I were to purchase one, I'm guessing this would be what you guys are talking about [...] I saw a few others on Amazon, ...
Don't see it mentioned here yet so be aware Comcast has an Approved Modem List --
Someone correct me if untrue but it's my understanding Comcast maintains customized firmware only for the listed makes/models and will not allow others onto their network.
In Nov.2011 I bought a used Ubee DDM-3513 from a Craig's List seller for $55 and have placed it onto three different Comcast accounts since. Been very stable.
Good luck! Let us know what route you took.