Currently the server is limited to one MRV stream only and I haven't noticed any performance problems on the server when someone is streaming from it.
How well does it work, farily well for the 1st shot at it. While the MRV client has been out in the H21's for a number of months DirecTV really hasn't been interested in issue reports, in fact their notes say they don't want MRV issues yet, only for the HR20 CE which just came out have they asked for issue reports. What I've seen is there's an occasional glitch in the video which results in some macroblocking for a second or two and the occasional audio dropout.
For some reason there's a couple channels that I use MRV on that have an occaisional problem where it looks/sounds like someone's hit fast forward for a couple seconds.
Both of these issues I've seen aren't due to my network since they also happen if I directly connect the client and server with just a cat 5 cable, eliminating everything else. Hopefully now that DirecTV looks like they're interesting in MRV issue reports these problems will be resolved.