The ONLY cablebox/DVR you can purchase and use with Comcast is Tivo. The cableboxes you see for sale online will not be activated by Comcast. You also cannot use DISH network equipment with Comcast.
I have 2 Tivos, and no Comcast box. My first cablecard is free, the second one is $7.45/mo. I pay no monyhly fee to Tivo, I purchased the lifetime service. That lifetime service has long since paid for itself. My Tivos are the only bright spot in my service with Comcast.