Well, the Viewsonic's in and it's doing pretty good so far! It's a very small and good-looking piece of kit. Attaching a USB drive to it, you can access it as a network share and add files in case your network can't keep up with the hi-def streams.
I only tested a few divx and mkv files on it, both streaming and from USB, and they all played perfectly fine (even files that caused the Xbox 360 through PS3 Media Server to go all screwy and out of sync).
I did notice an issue where it suddenly stopped connecting to the network shares (though uPnP still worked, strangely), and wouldn't play any mkvs - instead just coming up "file not supported" - but I just restarted the box and it worked a treat after that.
iPlayer worked first time, and watched "How TV Ruined Your Life" with no issues whatsoever, though as far as I'm aware you can't pause/rewind/ff iPlayer on this box but I didn't try that.
Over the weekend I'll be hooking it up to the amp via optical to test the surround capabilities. I did get some jerkiness from a 720p Blu-ray rip when streaming on the network during a fast-moving sequence in a forest, but I assume that's more down to my network speed than the box itself. I'll test the same file off USB tonight to be certain.
All in all, at just about £60 I can't complain at the moment. Does everything I wanted it to, even if there are a few little niggles.