I own this device, it is single tuner ATSC/QAM w/cable card so it can decode over-the-air digital (HD) and analong broadcasts and both analog and digital cable. It's basically a HD DVR cable box that also records OTA. The biggest difference between this box and the one supplied by your cable co. is that it doesn't use your cable companies Program Guide (cable card is only unidirectional at present) nor can you order PPV or VOD.
The HDD500 comes with TV Guide On Screen (TVGOS) which is integrated into the unit. TVGOS downloads guide data from various OTA and cable sources (typically your local PBS analog channel) and seems to work ok after a few kicks to the face. I have a lot of experience with this stuff...but if you're not into gadgetry then TVGOS may be a bit tricky.
As I said the TVGOS works ok once you get it setup, but please understand it does not have a slick interface like the Dish HD DVR and HD Tivo. It's my opinion that the HDD500 with TVGOS is similar in function to any of the cable HD DVRs I've used. Anyway, check out these links:
http://www.avsforum.com/avs-vb/showthread.php?t=537711
http://tvgos.theabsolutenuts.com/
http://pvr.hidyman.com/
BTW, it is not capable of tuning satellite (Dish, DirecTV, FTA) nor does it have any inputs to offload recording from your satellite receiver. It is strictly cable or OTA. We record tons of over-the-air programming (CSI, Cold Case, etc.) and we try to use it instead of our Dish Network HD DVR since we're always fighting for space. If there's something we really need to save, we just dump it out to our Panny DVDR.