I made an account to answer your question... thats how much i felt i needed to answer
OK, On my old cable box I was getting drop outs and the likes on the digital channels, had comcast come out and they told me that the line split to many times before the box so it was having trouble getting a good signal, the tech ran two dedicated cable lines (one for the box, one for the modem) and the problem went away. Now given that the DVR box processes almost twice the information, you may have the same problem, but with the non-DVR box, it didn't affect it because it used less information. So I would either call comcast and have them run a dedicated line in for the cable (They do it cheap but are not very clean about it), or call some company that will run cable and have them do it, this will cost more, but you can have them do it in a clean and seamless fashion. Comcast just drilled two holes in my wall and ran cable, no jacks... ugly but it works well.
As for the long load time on the guide and when changing channels, same thing, not only are you streaming the content, but information about picture format for HD, captions in a variety of languages, and writing info to the hard drive rather than either ram of flash.
The other thing to keep in mind is that, at least in SE Michigan, the dual tuner DVR boxes have only been out for a month, there still newish so there is going to be problems with them like any newer tech.
I got my box yesterday, hooked it up and immediately called comcast to complain, the tech explained it all to me and I am willing to live with the draw backs in exchange for Digital audio (Desperate Housewives in 5.1
), DVR, and HD content.