I have a Seagate FreeAgent Theater Plus, though I think they're called GoFlex Theaters or something like that now. My buddy has a WDTV Live.

I like his better overall, except for one factor - the Seagate will continue on to the next video when it reaches the end of the one you're watching, which is great for marathoning TV shows; the WDTV goes back to the menu screen at the end of each file.

But the WDTV does have less issues with certain files, and I'll often have to remux MKV files for my Seagate using mkvtoolnix - 99% of the time, the video and/or main audio tracks don't have their default flags set to "yes" or "default," in which case the video and/or audio won't play. Unfortunately, it also doesn't recognize default or forced subtitle tracks, so for some things I have to remember to turn the subtitles on (usually I'll just burn the forced subs into the video when I rip it in Handbrake in the first place, though).