Most important, you can actually purchase your own modem and not pay $15 a month for there modem.
The modems are compatible but you need to make them program it through their network, it was not easy but I did.
Like he said, when I was using Comcast cable and even DSL here in Maine, I always purchased my own modems. Yes, you have to spend some time on the phone with them, but it saves money. And no matter what BS they tell you, the output from the modem can be used with ANY wireless router, at least for internet.
YES, as Jack said, it takes your time for you to get on the horn with these folks to make them work; they won't work out of the box. AAMOF at one point the Comcast modem was exactly the same as a Motorola one, just had different decals. For internet only, there are even combo Modem/WiFi Routers for cable or DSL. HOWEVER, when you start talking TV, VOIP Phone & Internet, these personally purchased Modems and/or WiFi routers may not be the case. If you have a Fiberoptic MUX that has a RJ45 Female Port, EVERY WiFi Router will work for internet.
In the case of "Wireless Cable TV", I have a separate WiFi network. I have Satellite, and for my Wireless TV, DirecTV has its own wireless network set up which I had to purchase and it took the DTV tech a good couple of hours to setup. It was not something I could do as the tech was on the horn multiple times with the Mothership along with using special codes and crap he had to key in.
The DTV WiFi router is plugged directly into my Satellite Receiver, and the one TV that's on the system has a receiver that gets it's signal from that WiFi network. All my TVs are linked to the one Satellite receiver so they have access to broadcast and recorded shows, along with all the functions that the receiver has to offer. Additionally, since the receiver is on my home Internet LAN, when the satellite signal tanks due to rain, snow, etc., the receiver gets complete broadcasting to transmit throughout the house.
I expect that the VOIP phone system could be mean being locked into purchasing/renting their equipment. No actual knowledge here since my security system locks me into a good old analog phone line...