They've posted a comparison review of the WD TV Live, Seagate FreeAgnet Theater+, Popcorn Hour C-200, Patriot Box Office and Netgear Digital Entertainer Live in: HD Media Player Battlemodo: Apple TV Killers
Specs wise, the Popcorn Hour scores high, with the Patriot a close second, but in testing, the WD and Seagate were given the win (a tie.) The Popcorn does well with a third, but got dinged on price ($300 compared to the WD TV's $150.)
I did find it amusing that though the first paragraph says "When Apple TV 3.0 came out, we were unimpressed" the third paragraph begins:
This isn't about photos and music. Apple TV is better at both of those than any of this stuff
I've said this before, but I've been tempted to get a WD TV or WD TV Live, because of it's ability to play video files without having to go through another conversion step as I have to with the Apple TV. But I do that so infrequently, that it doesn't bother me enough right now...