I tried to use Windows 7 media center as a DVR for a while, but it just had too many bugs to be practical. It had some very nice features, but it felt like an unfinished product, and none of its problems were ever fixed.
I used it with a cablecard tuner on digital cable. Some channels had no sound, and some channels were offset by an hour in the guide, so it always recorded the wrong thing on those channels. If it tried to record a channel that it couldn't lock onto, it would generate an error message. Sometimes, it would have 20 or 30 stacked error boxes that I had to click "cancel" on before I could do anything else. If I changed anything about the tuner setup, it would forget my favorite channels, and the QAM to to guide mapping that I had setup manually. Now that MS has officially abandoned media center, there's no chance the problems will ever be fixed. In the end I bought a Tivo S3 with lifetime activation on ebay.