I guess I'll chime in since I have a WDTV Live in our guest bedroom, and I've used Media Monkey in the past.
-We have a WDTV Live connected to our main music/HT system that gets all critical listening/movie viewing. The PC is connected to an old Pioneer receiver via stereo analog RCA connections but is primarily used for organization etc rather than actual (critical) listening. I'd like the ability to "push" (via DLNA/UPnP) playlists to the WDTV and have the same audio playing on the PC and WDTV for parties etc. MM has some challenges here, does JR?
As has been mentioned, latency over DLNA is the real issue here. There will be noticeable lag if the rooms are close together, and it gets worse as the tracks change. If it gets too far out of sync, MC will pause and sync it back up pretty well. So in my experience playing a playlist: after a track change you get ~30-seconds of very lagged playback then ~5-seconds of pause, then it plays back together pretty nicely again. This really isn't acceptable for me hearing this constant out of sync then long pause at the start of every track.
If you are playing all-day-DJ and manually changing each track, there will always be latency. Sometimes as much as ~2-seconds and sometimes as little as milliseconds, but there will always be some. Unfortunately, it is not enough for MC to auto-correct either.
The only thing I've found to work well for multi-room playback over DLNA is to make albums or play lists into one track. MC is actually in sync the best when it auto-corrects, so I play one track, then have it auto-correct after it switches over at the beginning of the album, and it stays pretty well in sync through out the album. Convert Format and Media Editor that come with JRiver make this easy to do, but you do need to plan ahead if you are going to create a playlist for a party. Additionally, you can't really have guests adding tracks to an ongoing playlist.
I don't see this as an issue with MC and WDTV Live, as much as it is an issue with WiFi and DLNA. I have to do the same thing when I trick the LG TV in our master bedroom into being a Renderer. Other multi-room play back systems like Sonos have latency issues as well. I have better luck running multiple versions of MC on 2-PC's, but latency over WiFi is still somewhat of a concern there too.
-I'd like to control what's being played on the PC and/or the WDTV via our Android phones/tablet (I believe JR has "Gizmo" for this, correct? MM looks like would I need something like BubbleUPnP?)
WDTV Live is a Renderer so you can control it via Gizmo or through MC on your PC; however, there are some limitations. As pcstockton mentioned, if you initiate playback with MC or Gizmo, you have limited use of the WDTV Live Remote. I typically do not switch back and forth between Gizmo and the remote (it is usually one or the other), but it is something you should definitely consider. Even something as simple as skipping a track ahead when listening to music will no longer work on the WDTV Live Remote. Additionally, you can not use Gizmo to link and unlink Zones. If you wish to add music to your other room, you would need to go to the PC.
The other consideration is that control through the PC or Gizmo can sometimes have lag due to DLNA. It is the worst when Zones are linked in MC (ie playing in two rooms at the same time) and when you give multiple commands in a row. Gizmo can be used as a remote in 2-ways: 1) from that Zones Playing Now screen or 2) using it as a Theater View Remote (where the screen basically acts as a track pad w/ a drop down menu for some additional commands). While I prefer the 2nd option when using Gizmo as a remote with my HTPC, I do not like it at all as a remote for WDTV Live. It is a little too laggy and giving it commands to skip forward or back (ie swiping left or right) means you are giving it multiple commands in a row which causes it to hang. When you use the Play Now screen, it is a bit more responsive, but there is some lag. For example, if you advance a track by touching on the playback status bar, the audio will skip ahead, but it will be ~3-seconds before the playback status bar moves to the correct location on your Gizmo screen. In other words, it is difficult to know exactly where you are along the timeline of the track.
I prefer the views I have set up in Gizmo and scrolling for navigating vs using the WDTV Live menu and remote so if I am looking for something to play and forget it, I prefer to use Gizmo. If I am going to be hopping all around, skipping ahead in tracks, etc, then I use the WDTV Live remote due to response time.
One last thing to note, I can not get videos to rewind or fast forward on my WDTV Live regardless of using Gizmo, a PC, or the WDTV Live remote. Audio works fine. They look like they are doing it, but then pause for a second and go back to the very beginning of the video. You mention this is not an issue for you, but in real life you would be surprised how much of an issue it can be. Imagine being 40-minutes into a show, you try to rewind for 10-seconds because you couldn't hear something, and instead it now starts over from the beginning with no way of advancing through what you have already seen. This could be an issue with my DLNA settings in MC, but I have yet to find something that works correctly (I'll touch on this a bit more later).
In regards to the Linux/licensing issue, since you are rebuilding your NAS, you may want to consider building an actual home server and using WHS 2011. Granted, $50 is $50, but a $50 purchase of WHS 2011 does give you some advantages over OpenFiler or FreeNAS. You could then use the same JRiver Windows license on your server and PC. And on your laptop, and the HTPC you eventually end up building (see below).
Some closing thoughts:
You mention you are not really interested in playing audio/video through MC on your PC, but more as using it as a server. I would suggest that you try it out, and you may end up rethinking that idea. You will be missing out on some of JRiver's best features. There is a reason the WDTV Live is in the spare bedroom, and the family room and basement have HTPCs. I have used a lot of media players, organizers, and servers over the years, and for perfect, seamless play back of any format, there simply is not a better program. Before I close, I'll cover a few of what I think MC's biggest flaws are; however, they are all out weighed by its powerful playback.
The only advantage Media Monkey has over MC is its ability to manage Windows file structure. You can use the Rename, Move & Copy features in MC, but they are not automatic like they are in MM, and they are not intuitive. MC is designed for you to organize and view your library within MC itself. I get around this by having the basics of my file structure already set up and using Tag&Rename to rename my folders. It would be nice if MC automatically created your file structure for downloads the way it does for ripping, but it is not there yet like Media Monkey is. See this thread for details:
http://yabb.jriver.com/interact/index.php?topic=85295.0The other issue I have with MC that I mentioned earlier is DLNA support. There are no set device profiles like you get with a program like Servio. There is also no real trouble shooting guide to set up your profile correctly. It is pretty much just trial and error to find the settings that work for your device. Response in the Media Network forum is also poor compared to other Interact forums. I still do not have video working the way I would like on either my TV or WDTV Live.
Lastly, JRiver's development team put a lot of time into developing powerful and technical features, but they do not always go back and "polish" them to be more user friendly or fully flushed out. For example, there is still a lot that could be done with Gizmo, Theater View, Zones, User Accounts, etc. I am hopeful that they will come back to them, but resources must be getting thin with Mac & Linux development. I understand why they do this from a business perspective (ie sell more licenses), but it can be disappointing reading things discussed but never really worked on.
As a father of twin 5-year old boys that is in the middle of a whole house reno, I understand time is limited. I do suggest you try the 30-day trial though. I have come and gone from JRiver 3-times now, and I have used all sorts of other software, but in the end, I still find it to be the best thing out there. The power and customization options really are second to none. Besides, now that your boys are 1, the tough times are behind you.