TiVo was the first DVR/PVR and by many reviewer's statements today is still the best. TiVo's Apple-like ease of use and graphical user interface made is a consumer favorite nearly overnight. Companies like Sony made TiVo DVRs for use with DirecTV but soon after Rupert Murdoch's News Corp (parent of Fox and owner of DirecTV) took over DirecTV - they soon dropped their partnership with TiVo for their own HD DVR that ultimately got MPEG4 feeds thus more HD channels.
In TiVo's darkest hours they made only a DVR for digital cable users, though they have re-inked a deal with DirecTV and will be releasing a new TiVo DirecTV box.
HomeTheaterReview.com's TiVo product reviews include:
TiVo Premiere HD DVR Humax DRT800 DVD Recorder with TiVo Service HR-10-250 DirecTV HD TiVo DVR