GameSpy has published their report on the titles displayed at Gamestock. Here's their assessment of Halo from the Xbox page:

Yes, it's Halo, one of the most eagerly anticipated games of the last two years. We already know it's coming first to the Xbox, but we finally got to see it in action. No demo movies -- we played it, live, on an Xbox prototype. Halo is a game of third or first-person futuristic combat against an alien race on a unique ring-shaped artifact of a planet whose secrets could alter the course of a galactic war. Driveable vehicles, realistic physics, and stunning visuals are Halo's biggest draw. PlanetXbox has the full scoop.

We'll let you know as soon as PlanetXbox's new preview appears.