A few things that matter:
1. The cable company may really screw up the quality of what is delivered to your home. This is completely outside of how well the cable box performs and may leave you really upset. Then you could have the same box in another cable district and it may look awesome. Or vice-versa. A lot is in the hands of the cable company.
2. If you are renting, then it is definitely worth taking a look at. Tivo can't deal with HDTV unless you have it integrated into a DirecTV receiver. So, that doesn't work. Also, all outboard PVRs really don't hold a candle to integrated PVR units.
3. I have the Scientific Atlanta HD/PVR box and it absolutely sucks compared to the Motorola boxes that I have seen and used. If given the opportunity to use a Motorola box I would switch to it in a heartbeat.
Audio and video is usually very good on cable from digital and HD channels (analog is still analog and isn't nearly as good). Motorola is definitely a step ahead of Scientific Atlanta in product design. I have not seen a PVR user interface that is nearly as good as Tivo... But, I have also never been really unhappy with the user interface that does exist. It is all much easier, and much better looking than my old VCR.
As long as you are renting the equipment, I would definitely recommend using it for several months over any other technology.