Well, after a little under four hours, and within 24 hours of it being released, I finished Portal, and I had a blast. Unfortunately, upon release, the game was bugged so that some people didn't hear any voices in the game. It was patched right as I finished the game. So, uh, at least there will be voices next time I decide to play.
Anyway, it's very good. It's not like anything else I've ever played. It's actually probably even worth the $20 that it costs if you buy it individually over Steam, even though it's four hours, but you're better off buying it in a package, of course, like The Orange Box. I already enjoyed Half-Life 2 Episode One, and I've still got Episode Two left, as well as Team Fortress 2, which I doubt I'll play too much of.
I assume that there will be a demo sometime, but perhaps not since it's just a few hours. If you're not already getting The Orange Box (why not?), make the effort to try it out on someone else's computer.