I haven't...
It could be a really good solution depending on what you want to do with it.
Generally I just want something that can output video in good quality... s-video minimum but if you have newer tv, DVI or component would be nice.
Also, the sound on laptops tends to be a bit lacking. I'm not sure how many come with some kind of digital out, but that's what I would want either in digital coaxial or optical output.
I haven't checked out the Qosis laptops, but if they can output good quality video and sound, have decent storage, then I'd be happy to use one. Just add a wireless mouse and keyboard (if you're far away) and you're pretty set... unless you want to do more than watch DVDs on it / do normal internet stuff (only with a TV and HT sound system)
If you want to go beyond that, Traire is right that you'd have to add some external equipment which would probably not be as good as you can get with a desktop solution (expecially in the cost department)