When you get digital cable, you have a couple of choices for service. (1) Comcast STB or DVR and (2) CableCARD devices like the TiVo HD, and TiVo Premiere and Windows Media Center CableCARD tuners such as the Ceton InfiniTV or Silicon Dust HDHomeRun Prime.

With a Comcast STB/DVR, you receive all the SD and HD programming that you pay for, plus access to Comcast's On Demand and PPV services. Comcast is responsible for troubleshooting problems and replacing dead/defective equipment. Cost varies by market, typical rental fees are $8 per digital box. Renting a DVR adds another $16 on top of that. Contact your local office for more information.

By using the Comcast STB/DVR, you are limited to whatever software and hardware capabilities that Comcast offers in your area. In most markets, Comcast uses Motorola DVRs that do not currently support storage expansion beyond the built-in 20-30 HD hour capacity. In some markets, Comcast uses Scientific Atlanta DVRs that may support external expansion with eSATA drives.

CableCARD devices like the 3rd and 4th generation TiVo's and Windows Media Center have a program guide with 14-days of program information; they download this guide information from their own servers using a wired or wireless connection to your home network. Advantages include superior DVR functionality, usability (ex: no remote lag, improved commercial skipping), up to six times the capacity (157 HD hours), support for external hard drives (up to 300+ HD hours), remote scheduling, and PC/Mac integration. Other TiVo features include multi-room viewing and the ability to download some SD and HD recordings to your computer. However, CableCARD devices do not have access to OnDemand or PPV. This limitation is changing somewhat as TiVo has worked with Comcast to allow OnDemand access via an app for the TiVo Premiere hardware in select markets, see this TiVo website for more details.
However any other CableCARD devices such as the older TiVo HD series or Windows Media Center are still unable to use OnDemand.

CableCARDs are essentially access cards; they plug into the device and authorize all of the channels you pay for. The Tivo's and Windows Media Center add-in cards support multiple tuners with a single CableCARD (M-CARD). Comcast will provide one free CableCARD as part of digital service, but additional CableCARDs will cost $1.50-$2.50/ea. If you have another Comcast STB/DVR in your home, or buy a second TiVo, you may be assessed an additional "outlet fee."

You do not pay Comcast STB or DVR fees to use CableCARD devices. The purchase of a TiVo typically results in lower monthly fees, but it takes years for that savings to offset the purchase price. People who buy a TiVo typically do so because they want the improved DVR functionality, usability, and storage capacity. If you decide to buy a TiVo, do so with the knowledge that that Comcast is not responsible for replacing dead/defective equipment (with exception to the CableCARD).

In mid-2016 Comcast setup a new self-service portal of CableCARD users:

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