Google TV, Roku, and JB ATV2 all support Plex. Google TV doesn't even really need Plex since it natively supports many of the popular formats like mkv out of the box. Google TV can play local USB connected files or stream it from a server without the need of Plex. Something Roku or ATV2 can't do. Roku can only play mkv via usb.
Then you have the whole Google TV search/internet. Picture in Picture, Google Market, and the awesome keyboard. I can press the search key and type something simple as "Atlanta Falcons game" and it will list the game if it's live, show me the channel it's on, and give me the option to change to it to watch it. Or if it's on later, give me the time and option to DVR record. Or if it's already over and it's on my DVR, let me play and watch it. Same for pretty much any show or events. I type a show and it will search my DVR, what's currently on TV right now, what's coming up, and videos of the show available on the web. What people are speculating the new Apple TV set with Siri will be able to do with voice commands, Google TV can already do with the keyboard and trackpad. And parts of it are brilliant. Parts need work. But it's freaking sweet overall package. Most people who pan it haven't even used it or given it a chance. Google TV is definitely underrated and misunderstood. Most of it is Google's fault and Logitech's crazy $350 launch price. But $99 firesale fixed the pricing issue and Honeycomb update fixed many shortcomings. Google TV version 2.0 will be out by summer of 2012 and will be running on ARM instead of Intel Atom. Eric Schmidt claims 50% of all new TVs sold next summer will have Google TV2 hardware built-in. That's a bold prediction but shows you Google TV is not dead.