In the demo you get to play through the first part of the single-player campaign. As Mathew Kane you must battle to clear a landing zone for the earth forces. Although fairly short-lived, you do get to wield your weapon around a Strogg installation and some alien air-defence trenches. As well as giving you a very small taste of what the single-player game has to offer, you also get to see how the multiplayer game holds up. You're given access to one map in two variations: The Fragging Yard and The Fragging Yard 1v1. The first is for multiple players, with the later for one-on-one duels. Game modes available are deathmatch, team deathmatch, and tournament.
You can download the 320MB demo over at 3D Gamers where you'll find both Windows and Linux versions of the game.