I have the multi-room DVR. Everything with fios is connected to your computer router via coax. Part of their feeds (video on demand, guide data, and widgets) all come in via the IP feed, e.g. Also the router serves the other set top boxes.
I have the MR DVR in my living room and an SD STB in my family room. When I want to watch something on the family room TV off the DVR, I can access the programming from that STB. The programming follows: DVR -> router -> SD STB. All commands from my remote move the opposite direction (SD STB -> router -> DVR).
Overall it's a nice setup and works really well. But there are some limitiations (currently - Verizon is working to fix some of these):
1) Even though you can hook up 6 other STBs, you can only access the DVR from 3 of those at one time.
2) The other STBs HAVE to be the SD STB. You can't feed MR DVR to DVR, e.g., or even MR to HD STB. The other boxes have to be the SD STB, even if you have the HD version of the MR DVR. This is one that Verizon is working on fixing.
3) Along with #2, you can only feed SD recordings to your other STBs. Again, this is something that Verizon is working on.
4) You can only watch recorded material from those other STBs. While you can do things like pause, rwd, ffwd from those other STBs, you can't manage recordings from them. I can't set up a recording on my family room STB, e.g., nor can I delete one from there when I'm done watching it.
5) There is a bit of a latency on the other STBs - button pushes take a few seconds to register - for obvious reasons - the signals have to go: STB -> router -> DVR, and the response has to come back the same way.
There's also another cool feature with the MR DVR. It gives you access to their Media Manager s/w. That allows you to feed digital pictures and music (and soon, video) from your PCs to your DVR. Unfortunately you can only access the material from your MR DVR - not from the other STBs hooked up.