Right now MRV is only HD DVR-> HD STB, the rumor is that HD DVR<->HD DVR will be coming soon.
As for your question of what advantages are there:
- Start watching a recording on one DVR and then finish watching it on the other DVR without having to record it on both.
- Got 3 or 4 shows you want to record at the same time, set them up on the other DVR and then you can use MRV to watch it on the other DVR.
You can't convert a HD receiver to a DVR but the current implementation allows a HD receiver to view a recording that's on a HD DVR. You can't schedule recordings, delete recordings, modify series links or the like. Each TV still needs a receiver, either a HD receiver or a HD DVR, this isn't like Dish where their DVR has a TV2 output that feeds a NTSC signal to a 2nd TV. When playing a recording using MRV it plays in HD if it's HD or SD if recorded in SD, plus if it's DD5.1 audio that will also be supplied via MRV.