Be warned, it's very short (it's not over until the screen fades, though; no spoilers from me how to get to that point), as it's basically a "fifteen minute tech demo." I recommend turning on the commentary; this appears in the form of bubbles that turn around as you get close, and if you [USE] them one of the devs will speak. Good stuff overall, and some nifty effects are used to show Valve's HDR versus the old stuff.
For my part, I'm not sure HDR is THAT good overall. Whenever you walk up to a light and turn the flashlight on, it appears too bright at first, and then quickly adjusts. Doesn't matter if you've just done it; you can repeat the process over and over. Sure, the game benefits from it, but it's not as striking a difference as you might think (of course, the Source material, ahem, was already quite good). Colors do bleed into each other pretty nicely, but it's hard to say if every player will like this new effect. For my part, it seems DOOM III's lighting system is still king.