I installed Plex on my laptop and on the GTV a few weeks ago. I have extensive music, photos and video libraries on my Laptop, and it took days for Plex to catalogue it all, and make the streams available for viewing or playing on the GTV, and it really didn't give me much indication of what was going on under the surface during that time. It does work, but I think the UI is very unintuitive and confusing.
Several days ago, someone on another forum put me on to the free Avia Android app, which can be installed on GTV, and other Android devices via Android Market. I now have it installed on the GTV, the Droid X, and on my ASUS Transformer. Setup was pretty easy, and obvious. The only point on the setup that had me confused initially was that I didn't understand I needed to go to My Sources and click on the device name area (not the adjacent checkbox) to see and play its contents. I had almost immediate gratification using this app vs. the confusion on Plex. I am easily able to view or stream media to GTV, from either of the Avia servers on my Android mobiles, or from my Synology server.
I can also push media from either mobile to play on GTV or on the other Android devices, and I can pull streams or copies of the actual files to either of my mobiles from any of the servers. I can't tell you which codecs are or are not supported, but so far, it's played everything I've tried. I'm considering uninstalling Plex at this point.