FWIW, I've tried Apple TV, a few Roku boxes, and a WDTV Live. The only one that plays back nearly everything I throw at it is the WDTV device. Interface leaves something to be desired (not Plex, clearly), but it's still acceptable IMHO. Of course everyone's going to have a different opinion on prioritizing the GUI issue.
Haven't had the opportunity to try out a Popcorn yet. Looks very promising(!)
Nothing beats a HTPC though. People forget that you can simply leave the desktop where it is in the home and run cables for IR blaster/HDMI video to the HDTV to make the HT space 'cleaner' per se, but gain the 100% customizing of what software/hardware you choose on the HTPC.