I have a Comcast Motorola DVR, works great, although its not the latest edition. The cable guy that set it up was ready to install the latest one, but after talking to him a bit he hooked me up with the older box (got it from another technician's van a few blocks away). I can't remember why, but he said the latest version lacked something. (hope that helps???)
Anyway, I'd recommend just talking to the cable guy. Most of them know what they're talking about.