Using the Plex app ($0.99) it can play MKVs at 1080p without transcoding (Plex are still working on some audio issues) but its a descent experience.
Personally I'm not a huge fan of the Plex experience, but it's better than native Roku experience, and Roku+Plex.
The WD TV Live is slower, and uglier, but has very good audio codec support.
i don't like plex. i have a wdtv and its ok. works pretty well and no jb needed.
maybe firecore can jb the fire tv and put infuse on there.