Ideally, I'm waiting for something thinner than a PC. However, where the audio streaming boxes are a fairly mature category, the video streaming boxes all have major flaws.
Either they don't support crucial formats (divx), they won't play ripped DVDs, or they don't play AC3 streams or subtitles. The ones that work reasonably well with video have poor or missing audio support.
If I had to pick one right now, it would probably be the Pinnacle unit. The hardware's pretty good, it does most everything you'd expect in an intelligent manner, but its server software sucks and is a massive resource hog.
I think that all of these devices will improve when Microsoft forces them to use WMP10 as the server software instead of the craptastic homegrown apps.