I had the HR10-250/Tivo DVR for a number of years and loved it because it was rock solid (ie. froze maybe once or twice, recordings were successful, etc). I added the HR20-700 DVR around May 2007 and have been happy with it since I received it. I wasn't an early adopter like some people here, so I think any stability issues had already been resolved by the time I got it. Like most people, I did miss the dual live buffer, but I've moved on because I actually like the HR20 unit (menu is intuitive, I like the guide, I like that I can still watch live tv/recorded programming while reviewing guide/menu, no more phone needed, online scheduling, etc). I have only had the unit freeze 2-3 times and that is acceptable to me (especially after using Microsoft's Media Center edition on my PC for recording programming the three years prior to my Tivo experience).
My biggest complaint with the D-DVR is actually with the remote control for two reasons: (1) I prefer the ergonomic feel of the Tivo remote and (2) I prefer the location of the pause/fwd/stop buttons on the Tivo remote because they are in a more natural location in my mind.
Earlier this year, I officially retired my Tivo box by having it replaced with a HR21 model -- which is nearly identical to the HR20 -- and I especially like the two Ethernet jacks on the back so that I can chain my two DVRs together using a single data port on my home network. Again, I like this unit so I always find it interesting when people report about all of the problems that they have had with the D-DVRs.
One last note, I work with some people that have the Comcast DVR and I hear nothing by negative feedback from them, so I'm very happy with what D provides.
Marty.