Comcast installed my 5100 today, here are some observations:
Installer said it was his third install so far. He brought a component cable (which also had L/R audio), but no optical. The component cable is a 3 Meter cable (I think) and all five cables are connected together into a ribbon (but you could zip them apart if needed, I guess). Install was quick, but standard wait (5 minutes) for dispatch to authorize the new box. After authorization, get HBO, ABC, NBC, CBS and FOX. Picuture quality with HD programming is nice, not quite up to the same level as my OTA channels, but at least there are no drop-outs like my OTA from time to time (Silver Sensor, so I've never seen CBS-HD and NBC has been VERY sensitive to antenna position.) The analog picture quality through the box is acceptable.
Motorola 5100 box does not have firewire ports at all. Also doesn't appear to have DVI, unless it is behind a black plastic cover under the component (haven't looked yet). It does have USB on the front and back (not active), Ethernet (not active), DD Optical (active). Box is default set to 16x9 output with 480i override for non-HD channels. MUCH snappier than my old DCT2024 with channel changing and locking. I did get a new remote, which is identical to the old one I had with my DCT2024, except it says "Comcast" on it.
I also have an AT&T Tivo Series 2, which I used to have connected to the DCT2024 via composite video and using serial control. Since the 5100 lacks serial control - I also didn't want the Tivo to prevent me from using the 5100 for HD viewing - I moved my Tivo off to regular analog cable.
Was $5.00 more a month for the box and $29.95 for the install. Otherwise, I'm still paying $61.99 for Digital Silver package that I had prior to HD.
Overall, I'm pretty pleased, but I do hope they add some more HD channels soon...